To me, it's pretty clear on what happened:
GameSpot whores itself out on a semi-daily basis to any publisher out there with enough cash. This, we've always known, and most big video game sites/magazines are pretty similar.
There are games out there that have a much higher production value and advertising budget.
GameSpot wants to cash in on this by attracting more casual readers by covering the games people have advertised some on TV and things like that. Makes sense, right? It's like a magazine giving cover time to a game that's advertised well. The people who have seen the advertisement are more likely to pick up the magazine than those who have. It's just good business sense.
So, GS presses their editors to cover these games to a larger extent than no name games, perhaps like Phoenix Wright, Katamari Damacy, any Atlus game out there, etc. because coverage of the larger games brings more readers to the site than coverage of games like Phoenix Wright, at least, initially.
GS editors, in this particular case, Gerstmann has been getting fed up with what's gone on, and became tired of covering mostly bad games with high advertisement budgets. His quality and zest for work declines, and he begins submitting things late, poorly written, poorly represented, and just overall, his production decreases. We can proof at least some of this: Gerstmann's video review of Kane and Lynch. Gerstmann showed up late, recorded with a messed up microphone, and never did anything to fix it. The video contained basic, level-one-type footage of the game, and the review was a verbalization of his written review. That sounds pretty shoddy to me. Also, his written review was altered to "Better fit the score." The score wasn't changed, but the written part was. While I have not read the review, neither before or after the changes, personally, I conclude that there were likely some poorly written parts, or the score would have changed.
As far as the timing goes, Gerstmann was fired because the Kane and Lynch review exemplifies the problems with his work. It's been in the decline. They needed him through the busy part of the release season, they didn't have enough staff, but once they made it through that congested patch, he was out of there. You can't just demote someone who's a face, though. People would wonder what happened. Now, by firing him, they are, by law, not allowed to talk about things. For all the public is supposed to know, he could have been taking bribes, stealing, or worse. However, because of the Kane and Lynch review and skins, the public construed their own ideas. The result? Gerstmann and Gamespot have to keep quiet on the reason and can't quell the rumors well. GameSpot can't do anything to prove why he was fired, and Gerstmann, bittered for being wrongfully fired w/o notice, seemingly out of nowhere, won't do anything to help GameSpot out of their predicament.
The employees don't know why he was fired, they aren't allowed to, so as far as they know, it was for no reason at all, so they're pissed off, clearly, so they won't really help out, because they want answers too.
Thus, we have the mess we're in now.