Ooops, it's not in the GameSys.ini , it's in the Male.ini ...
Look for, under [Male/Shirt] :
Model = Tshirt01 1-11 Tshirt02 12-16 Tshirt03 17-20
and if you add normal 21st T-shirt, change it to :
Model = Tshirt01 1-11 21 Tshirt02 12-16 Tshirt03 17-20
Tshirt02 is the Shirt, & Tshirt03 is the sleeveless T-shirt.