I cannot get Gabriel into Mosely's office. The sergeant comes back almost immediately. I read somewhere you can wait till he falls asleep, but I find he nods, puts his head down, then lifts it up again straight away.
I am truly stuck!
Hope someone can help....
Just finished this game for the first time I'm ashamed to say. Great story, it definitely deserves the hype.
As for your issue, I remember having that same problem for a few clicks.
Just time it as you see him initially nodding and click the door. Watch him, there's a moment where he hangs his head down just a tad longer than the first couple nods.
That's the moment you're able to open that door and get in without trouble.
As previous users stated I'm sure it's due to a cpu speed issue. I tried turning down the game speed in-game with no luck, just had to get lucky on my clicking :)
Enjoy the game.