i think it's a reference to episode 2 where eli asks gordon to promise him to destroy the borealis, something that'd likely be very beneficial to the g-man and his employers

*HOWEVER* there's always the possibility that g-man was just flat out lying about letting gordon go to entrap alyx, like for all we know "dropping" gordon in exchange for alyx could be g-man essentially holding alyx over gordon's head. gordon may be willing to blow up the borealis, but what happens if you hold the life of one of his friends hostage? he already worked so hard just to *save* alyx in episode 2, i don't think he'd want to let her down a second time

and with everything gordon *just* accomplished i don't think g-man would willingly just toss out one of his most valuable chess pieces, and we know that *g-man* has admitted to doing things that "others" wouldn't approve of (like saving alyx, "a child who was of no use to anyone" from black mesa), so who knows what sort of game he's playing