GunGame was changed to not support teamplay in v4.2. The wiki is out of date. You can manually modify the gameplay's .py file to allow for teamplay by changing the line:
return GEGlobal.TEAMPLAY_NONE
in GetTeamPlay( self ) to
return GEGlobal.TEAMPLAY_TOGGLE
However, this is completely untested and may cause issues with the gameplay in addition to adding a (MOD) tag at the end of the gameplay. In modes that support teamplay, you can remove the line
GEMPGameRules.SetAllowTeamSpawns( False )
To enable teamspawns (for instance, Uplink is teamplay-only but has teamspawns disabled). Teamspawns is when one team spawns on one side of the map and one on the other side.