Honestly? He’d probably reject him at first. Especially because of all the times Soos has kinda done flirty advances on the show— whether he realizes them or not— and Stan pretty much has went “Yeah, can’t deal with this right now”. Though, in the show, I think GS pretty much thinks Soos is an idiot and that’s probably why he’s cool around him, rather than thinking Soos is really hitting on him. But to outright be told? Yeah, mega awkward. Maybe even to the point he can’t be in the same room with the guy. Of course he doesn’t fire him either— where else will he get practically free labor? That’s just smart money right there. He’ll just suck it up until it goes away, or until Soos stop being more of an idiot than usual.
Which I think would make for an interesting story of Stan having to work around the awkwardness to get used to being cool around Soos again. And then them being cool, to maybe Stan starting to like him back. Though, I bet there would be lots of denial. Maybe not so much from the gay factor— though i do kinda get ‘no homo’ vibes from him— but more from the fact… it’s Soos. Who would openly admit to being in love with Soos? (His logic, not mine.)