Check out the DOM gametype in UO, Earth.

As for "explaining" how to add a gametype...

sorry.. you'll have to figure that one out yourself mostly.. because a gametpye script is rather large and contains various functions.

as is - BAS will never occur in cod2 as the gamestate is way too small to support it..or vehicles for that matter

the gamestate for DOM might be small enough that cod2 could accomodate it, but the spawns need the logic hardcoded, and that is done on the engine level, not entirely through script

you could accomplish adding the gametype, but it would work like the dom and ctf that were developed for cod in that eventually the enemy would spawn in your captured or secured locations, simply because your spawns had advanced - thus they would be spawning behind you

Well.. in the CoD gametypes there is usually a fuinction called "Spawnplayer()". You MIGHT be able to alter the spawnlogic a little bit with that. But.. as Stryder said.. it will ususally happen you spawn in weird locations. As with every CoD gametype :p

since CoD2 works basically the same the same will happen.