I miss the point of the g-man and i think that there is too much little about the aperture. The only way that his behaviour can fit is that the g-man have created all this (the incidenti in Black mesa, the combine, the vortigaunts) just grab the Bootstrap Device from the borealis and be himself, like all the story is just a loop so he can continue to jump back and forth the timeline. I think that glados is stuck in the aperture for the same reasons, and maybe if she can get the hands on the same device she can finally be free. Again, maybe this is why the g-man is never seen in portal, because all he want is that the aperture create that device and glados stay out of the game. Just my two Cents, sorry for my English.