I'd trust Gay to guard Montie, Dirk, Ibaka, and Green, so that's half. Go big with JT or his equivalent replacement against Z-Bo, Griffin, LMA, Splitter (although Pop sometimes goes with Diaw for major minutes and Gay should fare fine against him), or Davis.
Against the west non-playoff teams? Gay would be ok against Morris, and whoever the Wolves, Lakers, and Nuggets are throwing out these days. Favors would be the only real concern.
Out east? Easy. I'd be worried about Gasol. Bosh when healthy (although they were playing him mostly at C until Whiteside emerged, Gay could do fine against their original starting PF, McRoberts). Maybe Monroe and West, and Millsap. I would have thought Nene, but last night showed otherwise. No one else, really.
So, Gay can't hang at PF against six or seven of the 15 other teams out West, and maybe five or six of the 16 teams out East. Not that big of a deal.