I seriously doubt GSP is coming back. But if he does, he has to enter his name into the USADA testing pool of athletes a full 6 months before he can be cleared to fight. Exceptions can be made of course. The rules state that any fighter coming out of retirement must make themselves available for testing a full 6 months before their return.
His name might be in the pool, but up to this morning, he hasn't been tested.
(This is not a PED accusation against GSP. Just an explanation of what a retired fighter has to do to be reinstated.)