there's also the unfairness of their arrangement where Parker refuses to let Gil uphold his end of the bargain. yeah Gil asked him to but in exchange for getting Parker to watch his weight, which Parker won't let him do. so Parker gets to eat his cake and cockblock Gil too.
Gil says the arrangement is his own fault, starts to say why saying "i..." then he hesitates and walter questions him, then he says "lars and i" dated. i think that's a deflection and the real reason is something else. then he says Parker is there to stop him from making the same bad decision. he could just be referring to getting back with Lars, but i'm connecting it to what Liz overheard about Parker's mistake he doesn't want anyone else to make. the only thing we see him go out of his way to stop people from doing is sexual advances, and elsewhere it's said Gil is a bad influence on Parker. so maybe Gil feels guilty for whatever Parker's mistake was