What I like in my GunGame maps:
- Elevation (2-3 "levels" is good)
- Well-balanced but preferably non-symmetrical
- Multiple paths to choose from, but not so many that you step into a room and have 5 or 6 directions to be shot from
- Maps that you can get "good at" as you learn them better -- ladders that require jumping to, places you can reach in non-intuitive or not immediately obvious ways
- Both long-range and tighter areas to accommodate all gun situations
- Most importantly of all: Not too big! This ruins so many potentially good GunGame maps
If it's not already obvious from the points above, I think the greatest ever is texture city advanced.