I could see a reason to fire the GM now, if only so that the new guy could evaluate the talent on the team. I'm not sure who you would replace him with right now, and/or whether or not guys you might want (like the guy from the Patriots) are under contract with their teams and unable to talk right now like assistant coaches are.
Regardless, it's really not the Giants way. A total housecleaning is in order, but that will come after the season. Far-fetched though it may be, there's still a chance we finish with a winning record... Dan Reeves' Giants looked pretty awful at 3-7, but I remember everyone bubbling about Reeves and Brown once they finished 9-7.