SetPlayerTeam will make players of the same team invulnerable to each other so just set everyone to team 0 (or don't set team at all) to make them able to kill each other.
Please don't post if you don't have a clue what you're talking about. gTeam is not a native system, it's a common global variable name which is pretty irrelevant to the OP's question.
As for disabling anti-teamkill, you just need to set them all to different teams (SetPlayerTeam(playerid, playerid) or never set them at all.