[MrViolence101]

Код:

COMMAND:showg(playerid, params[])
{
    new Player;
    if (sscanf(params, "u", Player)) return SendClientMessage(playerid, 0xFF0000AA, "Usage: /showg [PlayerID]");
    else if (Player == INVALID_PLAYER_ID) return SendClientMessage(playerid, 0xFF0000AA, "Player not found");
    else
    {
	    if(PlayerInfo[Player][PlayerGang] != -1)
		{
			new msg[50];
			format(msg, sizeof(msg), "{FFFFFF}Gang Name : {%s}%s {FFFFFF}", Gang[Player][GangColor], Gang[Player][GangName]);
			format(msg, sizeof(msg), "%s\nGang Tag : [%s]", msg2, Gang[Player][GangTag]);
			format(msg, sizeof(msg), "%s\nGang Score : %d", msg3, Gang[Player][GangScore]);
			ShowPlayerDialog(playerid, DIALOG_GCP, DIALOG_STYLE_LIST, "Player Information", msg, "Chooose", "Cancel");
		}
	     	
			
      }
    return 1;
}

Your local variable 'Player' is the same as your enum name. Change that.