if I have some time today, I might start on gb, it's really easy once you get it down.
If you do decide to do this, the entities you should add are:
NeutralSpawn - you put them where you want the spawn locations to be
NeutralSpawnZone - you put them around NeutralSpawns to determine its zone; determines the spawn priority
HealthVial - heals 5 hp
HealthPickup - heals 25 hp
Water - self explanatory
Acid - like water, except with damage over time while inside it
I haven't programmed ladders or drop down floors yet. Well I have, but they haven't been playtested thoroughly.