According to the reports, the layoff came after Kane & Lynch publisher Eidos took issue with the review and threatened to pull its considerable ad contract. GameSpot's front page is currently almost entirely re-skinned with Kane & Lynch imagery.
The rumor began bouncing around various industry circles over the past day, and this evening was reported by Kotaku. Popular webcomic Penny Arcade posted a strip about the alleged incident tonight, outside of its normal publishing schedule.
Shacknews can confidently confirm via its own sources that Gerstmann was indeed fired yesterday from his position at GameSpot. The source declined to comment as to whether the firing was directly related to the reported Eidos situation, but the circumstances are suspicious at the least.
Gerstmann had been an employee at GameSpot for about a decade, and took the place of former executive editor Greg Kasavin when Kasavin left the company to work for Electronic Arts as a producer.
Update: CNET employee Tim Tracy has posted what appears to be a farewell notice on his GameSpot blog. Until July, Tracy was an editor at GameSpot, and since then has been working for CNET-owned MP3.com. If indeed Tracy is departing, it is unknown if it is related to Gerstmann's dismissal.