Like the Faash, they are an interesting case study as to how the Imperium could have turned out if circumstances had been differently. Where the Faash were like the Imperium in that they had Emperor with a vision of human purity/supremacy, but who was mortal and completely eschewed transhuman sciences; The Gardinaal had raised their own bloodlines of soldiers, as well as utilizing psykers, but were led by a group of "immortal" High Lords rather than an immortal Emperor.
"A really efficient totalitarian state would be one in which the all-powerful executive of political bosses and their army of managers control a population of slaves who do not have to be coerced, because they love their servitude."
- – Aldous Huxley, Brave New World
Though the Gardinaal were technologically advanced, they had chosen to abandon warp travel at some point during the Dark Age of Technology, preferring to remain in their own solar system. Even though they knew other races and colonies of humanity existed, they had effectively been isolated for many thousands of years.
The Gardinaal home system had eleven planets that were heavily industrialised and extremely heavily populated; One world alone was said to have 100,000,000,000 people which was supposedly greater than Terra at the time. They clearly held some STC designs as they were capable of building their own Thunderbolt Fighter craft for use in their air defense forces, they were capable of building large walker constructs, and they also had beam technology that was more sophisticated than that of the Imperium.
However, what was most interesting about their technology and society was how they had adapted the genetics of their citizens. Due to the extremely high population and no trade from outside sources, resources were at a premium. Therefore the entire population had been subjected to eugenic manipulation and psychological conditioning. Every person was designed for their role over thousands of generations of selective breeding and separated according to a Caste system like something out of Aldous Huxley Brave New World, with the smarter castes occupying higher positions, while the dumber castes were happy in their ignorance. The Gardinaal leaders were aware of genetic engineering (one was suitably impressed upon encountering an Astartes for the first time, and was able to relate to the notion of a Primarch) but they had either implemented it into the castes so far in their history it no longer mattered, or they had chosen not to waste resources on it and stuck with the long route via pure eugenics. The castes were very different from each other to be almost unrelatable, the "ogreish" Warrior-Caste were raging testosterone junkies, while the Enforcer-Caste were emotionally disconnected. There was even the Void-Caste members who had hands instead of feet (much more useful for zero-G) and there was a Consular-Caste who were bred for their psychic ability, but they all remained effectively human.
This was on top of the conditioning and propaganda that each person was exposed to. Even though the lower castes were a bit dim and ignorant, obedience to the state was absolute; all it took was a command from a senior and the lower castes would throw themselves at Astartes warriors without hesitation. Every person had his worth, and once their worth to the state was exceeded by their burden they readily accepted euthanasia so that their resources could be recycled and reused elsewhere. It is not exactly described what happens to these "Expired", but one of the Consular-Caste admits it would be an honour to be interred inside one of the High Lords; so whether this means being eaten, or having their organs and cells harvested and distributed is left to the reader to decide.
On the topic of those "Immortal" High Lords: they were nothing like the Emperor, nor were they Perpetuals. They were effectively Uber Dreadnoughts, being kept alive by machinery even after their bodies had failed them, though they had remained in this state for many thousands of years and presumably not have gone completely mad or unresponsive the way that space marine dreadnoughts are often expected to become with advancing age. One high lord was described as being one and a half times the size of a Deredeo Dreadnought with three times the firepower, though they were capable of changing chassis and occupying smaller more discrete forms where necessary.
The Gardinaal had only one encounter with outside races, as they were invaded by Orks 2500 years before the Great Crusade rolled along. The Gardinaal had the military might to see off the Ork invasion successfully, and the victory was used in ongoing propaganda to perpetuate the notion that they were undefeatable. The soldiers would maintain this even as they were losing to the Imperium, and it clearly caused them some mental dissonance.
The Fall of the Lords of Gardinaal
When the Great Crusade rocked in with a fleet of Ultramarines supported by the Thousand Sons they noticed the heavy industries of the eleven worlds and wanted it for the Imperium. So first they tried to negotiate.
However the High Lords of Gardinaal considered the notion of a galaxy spanning Imperium to be a gross exaggeration, and believed that the Expedition Fleet that encountered them numbered the entirety of the invading forces and stalled for time while they readied their military. When the Thousand Sons realised that they were also attempting to use psychic methods to drag out and influence the talks, they immediately withdrew and the Ultramarines invaded by encircling the capital city.
What the Imperial forces did not expect was for the Guardinaal to respond with a massive saturation campaign of nuclear warheads, wiping out a demi-legio of titans, 500,000 Imperial army dead, and 856 Astartes rendered incapacitated, not to mention the loss of gene-seed, in addition to causing massive collateral damage to their own population (which they inevitably wrote off as inconsequential) which also rendered the area irradiated for decades.
Into this arena comes Ferrus Manus, trying to prove himself capable of being named Warmaster by taking command of the remaining forces as well as bringing a contingent of Emperor's Children along with him and aiming to complete the compliance before Roboute Guilliman arrives with an even bigger force.
A more conventional war was fought. And although the Gardinaal have the advantage of numbers and their own technological equivalency, they are mostly untested troops against the mind of a Primarch and the physical superiority of the Astartes. The High Lords of Gardinaal come to the realisation that maybe the Imperium is as big and as powerful as the envoys had described and once again attempt to sue for peace and offer a surrender.
This is where the various sources differ on the completion of the campaign. The account from Forgeworld in the Horus Heresy rulebooks is perhaps the more sanitised account, where Ferrus Manus refuses the surrender and continues the campaign with the Ultramarines practically begging to reclaim their honour by taking the enemy citadel, which the Primarch grants them.
However, in the more detailed Ferrus Manus novella, the surrender is just another ruse for a High Lord to get close to the Primarch with an assortment of hidden weapons the Imperium are unable to detect, a psyker consular for influencing the Primarch's mind, AND a willing suicide bomber to clear out any Astartes that attempt to interfere.
The assassination attempt on Ferrus Manus was enough for him to get pissed off and take his anger out on Akurduana, the commander of the Emperor's Children contingent who had been consistently one-upping the Iron Hands since his arrival in the expedition. The Primarch challenges him to a duel and breaks his rib plate, before announcing that he doesn't claim worlds, he conquers them, and readies his forces to give the Emperor ten worlds instead of eleven.
When the commander of the Ultramarines contingent protests that this is against orders and the intent of Roboute Guilliman, Ferrus Manus overrules him and commands the Ultramarines to make the first assault after he has spent some time blasting the city from orbit.
When the High Lords realise that they are about to lose again they attempt to revert to their first tactic of nuking the enemy and everything else in the vicinity, aiming directly for Ferrus Manus after he makes landfall. But this time the nukes are stopped as Akurduana performs a suicide run of his own on the Gardinaal command centre, effectively neutering any remaining resistance the Gardinaal have to offer, and the war ends.
The sanitised account indicates that the planet's infrastructure was left intact and that within a few years they were a model of Imperial compliance. However the more detailed account explains that the Rage of Guilliman was practically volcanic when he finally did arrive the day after compliance was completed. Not only for the state of the world that was now going to need rebuilding, but also because of the poor treatment of the Ultramarine contingent that had practically been reduced to nothing, and the two brothers barely even spoke to each other at the victory parade, having to go through Fulgrim to do it.
Ferrus Manus then stated that he had no longer any desire to become Warmaster, but would support whoever got the job.