Just to clarify, the game being asked about here is gpFINALwar not GPWars... as far as I'm aware, GPFW was only ever released with the built in campaign, and the capability to make more. Someone did start a Dune campaign, but I don't remember seeing it finished. Matkepon made it, iirc; he might know for sure if anyone ever took advantage of the campaign making abilities of that oh so fantastic game.
GPWars is the Advance Wars clone by Aquafish, and may or may not have had other campaign modes for it; I never played it.