In early editions of 40k, what was then called an Ork "Gunwagon" was an Ork vehicle commonly used as a transport. A basic Gunwagon consisted of a powerful engine and a mounted Big Shoota on a chassis. A Gunwagon only had a seat for the driver, but hand holds were provided to transport Orks over short distances. Much like other Ork vehicles, the Gunwagon was a highly modifiable platform, and could be armed with a Zzap Gun, a Kannon, or a quad Flak gun while still retaining its transport capabilities.
In addition, the Gunwagon could be up-armoured to protect the driver and gunner, and also mounted spike armor for extra stabbiness if push came to shove.
It is safe to say that the role of the Gunwagon was akin to a Space Marine Razorback in both function and usage. Like the Razorback, the Gunwagon acted as an IFV (Infantry Fighting Vehicle) of sorts, meant to help both carry a modest amount of boyz while hammering suppressive fire towards enemy entrenchments. Despite the name, the oldschool Gunwagon was big enough to only mount one pintle-mounted weapon. So there wasn't really that much GUN in Gunwagon. This remained true until....
2009 AKA the Year the Gunwagon got a Buttlift
The old 90's-era Gunwagon has long been removed from the game. For a number of years Ork players didn't actually have a dedicated Battlewagon/Gunwagon kit, but in 2009 a new Battlewagon kit was finally released. What is now called a Gunwagon is therefore actually a completely different vehicle that is classified as a variant of the reborn Battlewagon.
This new Gunwagon features an impressive array of weapon options in contrast to the relatively disappointing variant in previous editions. Whereas the old Gunwagon could mount one pintle-mounted weapon, the new Gunwagon mounts a turret-mounted main weapon and up to four Big Shootas. It may also be equipped with additional options such as a Lobba, Stikkbomb Chuckas, Grot Riggers, and even a Grabbin' Klaw and/or a Wreckin' Ball. Thus, this is a vehicle that actually earns its name compared to its predecessor. Moreover, it also includes a built-in periscope that lets it fire the main gun twice if it moved less than half of its movement characteristic or if it didn't move at all.