This quest can be acquired after completion of the main Feros mission if the colonists are not killed but it is impossible to complete in this context since the elevator required to access the prothean skyway is disabled on main planet story completion.
EDIT 26.05.2021
I reworded the title to make it clear that the quest is only impossible if you start it after finishing the main quest.
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.
agreed - I didn't acquire this quest until after bumping off the Thorian, but like yourself was unable to do it because the elevator to the skyway no longer operates.
IF this quest is ONLY possible to do if collected as a quest BEFORE going across the skyway, then it should no longer be offered as a quest (which can never be completed) AFTER the main quest is done.
The answer is for the elevator to not be locked after doing the main quest.
There is a 3rd party mod on Nexus which unlocks the elevator to enable skyway access so you can go back and do this, but not everyone wants to install mods.
Despite apologists who say you should just pick up the quest before going across the skyway so it therefore is not a bug, I consider that it IS a bug - it's a breakdown in logic. The bug is that you are even offered this quest (which is now impossible) following the main quest completion.
The simplest bug fix is not to lock the skyway elevator.
