i don't think he's slept with anyone, i'm believing Gil when he says he's a virgin on his day 7. when Walter doesn't believe it, Gil is quick to take it back because he wants to be seen as a sex expert. to me this feels like he wouldn't say he was a virgin if it wasn't true. And later when Gil is teasing Walter about it being his first time at a hot spring, Lars teases Gil about being a virgin and his reaction confirms it to me. Lars also sounds surprised when Gil said it, meaning they never had sex and he never found out when they were dating (which is weird, but maybe Parker was cockblocking him even back then, despite their arrangement sounding more recent).
Herbert doesn't say that Gil is a family destroyer, only that he has a certain reputation and he's someone to stay away from, it's probably due to the rumors surrounding Gil rather than anything to do with Edgar.
to get these scenes all you need to do is choose deny when gil says you already agreed to sit with him on the bus, then these scenes happen later.