A few ways to do this. First, you can use
this script to assign it to only certain maps types (by prefix).
Second, you can use map cfg files. To do this, create a folder inside your ..cstrike/maps/ folder called
cfg . Inside that folder, create a file for each map you have on the server that you want GunGame on. Inside each file, put
es_load gungame, and inside your ..cstrike/cfg/server.cfg file, put
es_unload gungame.
[shameless plug]
The third, if you also need/want a good VoteMap script, you can use my
LooneyMapVote script. With it, you can have a vote on gameplay, then on nextmap. It's a very versatile Map Voting script. Just follow the instructions on it's forum page, linked at the bottom of the ESAM page, as I haven't fully updated the ESAM page yet.
[/shameless plug]
Satoon