I think issues regarding drug use are precarious ground for the NFL. PED's are one thing because they arguably injure athletes, cause potential harm to other players and give a player a competitive advantage.
If Gregory does have bipolar disorder and probably other diagnosed disorders, it is arguable that they may be able to file suit against the league under federal law, including the Americans With Disabilities Act ("ADA").
That's not my area of legal expertise, but at some point this "policing" by the NFL is probably going to be tested and it may end up opening up the NFL for significant liability. It just has to be framed the right way and has to be pursued to the end. The NFL may have an exemption under antitrust law but that's not a blanket exemption from all federal laws.