On 'You're The Worst', publicist Gretchen Cutler claimed that she did two of the Baldwin brothers.
In case you're not familiar with all of them, here they are:
Alec Baldwin (born 1958)
Daniel Baldwin (born 1960)
William Baldwin (born 1963)
Stephen Baldwin (born 1966)
[The pictures are not in order.]
She didn't specify the brothers involved, and I could see Jimmy becoming obsessed with trying to identify which ones. The guessing game could even become a recurring theme with quick references, sub-plots....
Maybe a few of those siblings might even show up in an episode which could stir up the speculation.
Since it was a throw-away joke that could be revisited (no matter how unlikely), I'm not about to lock in any definite answer. Alls I know is that Stephen Baldwin seems the most logical.
But as for the other Baldwin she mounted?
I think the ultimate payoff would be for Jimmy to find out that Gretchen had sex with Adam Baldwin who isn't even related to the acting dynasty of Massapequa!