Some quest in the games will lock/being not doable if you don't do them at the right time.
This specific quest makes no sense after the Geth are defeated, because Gavin just can get over to the building and get the data himself, the threat is gone.
So I think it is an oversight that you still can obtain the quest after the main mission is done, but as it was said before, if Bioware will fix it I would asume they just prevent players from obtaining it at all.
The immediate threat is gone but it would still be a perilous journey for him, what with the skyway being in a such a state of disrepair and all. I'm more baffled as to why the game wouldn't let me get the quest from him at the ruins. I think I went back to get it after my first visit (but before doing much else, I think it was after the reunion moment that I tried), but I couldn't talk to Gavin for some reason.
