It's just the angle of her arm. The tattoo is smaller than people think it is. In a pic from a few hours earlier of Gaga with fans its clearly visible.
GaGa is a walking talking experiment on the psychological effects of pop culture/fame. Whether or not this is Stefani Germanotta is irrelevant, it's still in the fashion of Lady GaGa. Soo lets face it she "is a lie" & we must respect the artistry of GaGa, its completely genius♥