[Aly]

You have to add something like this to OnPlayerSpawn:

if(PlayerInfo[playerid][pMember] == 1) gTeam[playerid] = 1;
or with skins: if(GetPlayerSkin(playerid) == 121) gTeam[playerid] = 1;
You have to do this for all the gangs.

After that you use OnPlayerTakeDamage like Facerafter said:

pawn Code:

public OnPlayerTakeDamage(playerid, issuerid, Float:amount, weaponid)
{
    if(issuerid != INVALID_PLAYER_ID)
    {
        if(gTeam[playerid] == gTeam[issuerid])
        {
            new Float:health, Float:newhealth;
            health = GetPlayerHealth(playerid, health);
            newhealth = health += amount;
            SetPlayerHealth(playerid, newhealth);
        }
    }
}