Expect huge amounts of derp and rage, punctuated by /tg/ extracting humor from it.
Gav Thorpe is one of the many writers in GW's development department. He is not Ian Watson. He is the one largely responsible for the grand mother bull moose of all mixed blessings that was the 4th edition Codex: Chaos Space Marines, as well as the transition from the golden age of Warhammer Fantasy into the literal End Times. The former was a huge subject of RAGE and winDread, compartmentalized in one work that has drawn both many supporters - and many completely justified haters. The latter was just shit.
He has since moved into writing for Black Library for a short time, working on almost everything that has anything to do with the Eldar (most notably the Path of the Eldar trilogy). Gav also wrote the script for the Warhammer Fantasy game Warhammer: Mark of Chaos.
Why the controversy?
To put it inoffensively and politely: Generally his writing is bad and nowhere near as good as far more prominent writers. In particular, he's the guy behind the gloriously bad and cheesy ending to Storm of Chaos. He was quite terrible at the time, and he's always shown a very erratic understanding of even the most "historical" characters of Fantasy and 40k, as well as a sense of mystery, which others like Matt Ward also seem to lack. His Dark Angels work is also a point of contention, with some claiming it to be even worse than the Blood Angels series by James Swallow.
Another thing that drew the ire from many a fa/tg/uy is his usage of neopronouns (ve,vis,ver etc.) in his recent works, seen as pandering to the SJW crowd.
While most of his writing has left a bad taste in the mouth of the community, his work on the 6th edition Warhammer Fantasy army books is generally praised. In particular, Hordes of Chaos (when all of Chaos, minus the Skaven, was one faction) and Dwarfs are beloved to fans of those armies. On the OTHER hand, he was the Loremaster for Fantasy at the time, and he was far from light-handed with it, even at what was arguably the high point in his career, and a lot of the goofy bits from that era were his responsibility - such as the aforementioned Storm of Chaos ending, where despite getting beaten like a red-headed stepchild on the tabletop, Archaon and his army manage to escape total slaughter by way of... running away and hoping no one notices.
And then there's the elves...
Among the best examples of his writing style's flaws on display is his infamous views on elves, specifically that "there are as many elves as the plot demands." In one story they can write off the death of a million during a grand victory, but in the next the death of a hundred is a tragedy from which their race will never recover. Either way they will always be a dying race (with no word on how the High Elves or Dark Elves replenish their numbers for these huge wars with each other). This is to say nothing of his requirement that Elves retain the traits of the Eldar (souls eaten by Slaanesh, never worship Chaos) and the constant need to retcon anything that doesn't fit his very 40k-centric view of Warhammer lore. Most can't figure out whether he's in bed with the Elves or with Chaos, considering he varies between jacking them off at each other's expense depending on the book. In the end he monogamously settled on taking it up the ass from Chaos, and in turn made absolutely sure every faction took it up the ass from Archaon.
Thorpe's trademark is his whiny sentimental approach to writing: it's a formula meant to instill a sense of grand loss in the reader, which only works the first two or three times before it gets irritating. Worse still, he applies the same methodology to the Dark Angels and Raven Guard. Thorpe was basically that morose emo kid who cuts himself and seemingly never grew out of it.
Chaos in the 4th Edition Codex
With regards to his contentious handling of the Chaos Space Marines Codex, it's worth noting that the very character-filled and developed army lists of the previous Codex: CSM were replaced with much-more-generic-flavor army lists. Thrope claimed in a number of interviews that there were plans to eventually produce codexes for each of the cult legions, including Death Guard and Thousand Sons. These did not materialise until almost 11 years later, while Chaos Space Marines did not receive Legion Rules again until 2016 (which lasted only for a few months). While defenders of Thrope's 4th ed codex often cite the notably unbalanced nature of the 3.5ed Chaos Space Marine codex and it's tendency towards monolists, a majority of competitive armies following the 4th ed book followed the same formula of Lash Prince, Obliterators and Plague Marines. At least the 3.5ed armies had consistent themes.
It also completely removed several if not all options for viable weapons, utterly buttfucking any possibility of running variant legions. If you were running a Night Lords stealth army, an Iron Warriors warband with additional heavy support, or an Alpha Legion cult strike force, you would find that the new rules simply removed those options.
Hell, several normal CSM troops were rendered fucking useless: Raptors and Obliterators are no longer hard-capped (Obliterators becoming a staple of competitive CSM armies, Raptors otherwise have never been good). Special rules for Word Bearer champions, Iron Warriors Warsmiths, and Alpha Legion cultists were all completely absent. Faction-specific armies didn't suffer anywhere near as much - except in one critical role:
Chaos lost all faction-and-Chaos-specific Daemons, and any army that relied on them - especially the Word Bearers, which could field more than any other force - was completely and utterly screwed, either for fluff reasons or crunch reasons. Keep in mind, pretty much all the Daemons were viable at one point or another in 3rd Edition. CSM didn't even get to keep the Chaos Undivided Furies, for fuck's sake. Chaos also lost Greater Daemons of all stripes, and all we got in exchange were GENERIC DAEMON PACK and GENERIC GREATER DAEMON which, while still useful (as many players made GOOD use of them during official tournaments), are nothing but a pale shadow of what used to be available to Chaos Space Marines.
Why did Chaos lose them? Because Gav Thorpe decided that the Daemons needed their own Codex and update. By and large, players refer to most of the new Daemon models that followed this Codex to be fail; the new "one boob only" Daemonettes are absolutely fail-tastic compared to the lithe and graceful-looking ones of the previous edition. Oh, think that's bad? It happened in Fantasy too, breaking up Hordes of Chaos into Beastmen, Daemons and Warriors of Chaos. Only the last was truly competitive, the first being fucked to pieces on release. The crunch is also a mixed bag in the new edition - updated sucktastic Chaos Lords, heavily-diminished setups using the Chaos Mark system, and more.
The end result is that most Chaos players will continue to loathe his very existence and long to drag his soul screaming to the Warp when his time is finally up. Granted, this may be a mulligan on Thorpe's part: while he's considered a bad codex writer and did fuck up at least a few other codices as well, his involvement in the 'Nids codex in particular was fairly light (plus Robin Cruddance took that opportunity to nerf the shit out of them, so not really Thorpe's fault).
In addition, Matt Ward later wrote the Iyanden mini-dex which has them win their battles, something Gav apparently cannot do. However, the Supplement also writes Iyanden as being incompetent assholes who didn't even listen to ELDRAD when he stopped by to warn them of their Tyranid-chow future, in a rare example of him not being a dick. It's fair to call it even.
Where he stands as of 5th Edition Grey Knights Codex
As a general consensus, most fa/tg/uys tacitly agree that Thorpe should probably stop writing for Black Library and stay the hell away from pointy-ear lore and crunch. Much of /tg/ has decided that Matthew Ward is infinitely worse than this guy can ever hope to be; Gav hasn't mutilated the canon, he didn't FANBOY OVERPOWER anything except Chaos in Warhammer Fantasy, and he's been around long enough without acquiring a track record of horrendous stupid outside of the aforementioned CSM neutering. There ARE some that believe him to be worse than Ward, as his codices are more often than not
somewhat playable friggin' powerhorses balancing on the edge of being absolutely OP, whereas Thorpe's books strip the associated army of its flavour.
As a BL writer, his obsession with the space elves seems to be connected with a near-total lack of understanding of how they work, with his Path of the Eldar novels' plots requiring practically everyone on Alaitoc to be a drooling imbecile to work and going on bizarre tangents about how Khaine is supposedly related to Khorne. You'd think someone so insistent on writing a given faction would at least have their facts straight about them and not make them all look like emotionally stunted retards.
He's also one half of the Kyme-Thorpe law which states 'all Dark Angel and Salamander books must be shit.' And then, of course, there's the whole thing with the End Times in Warhammer Fantasy and the transition to Age of Sigmar...
All that said, some at least believe the man can write half-decently on occasion, unlike a certain someone. He does an awesome job of portraying Ortan Cassius and the Ultramarines in "Catechism of Hate," and he wrote the origin story of Eliphas the Inheritor. His novel for Age of Sigmar, "Warbeast," is also pretty good too. His worst work bar none is probably writing about the Dark Angels chapter.