Yea, the Bay Area is the highest density of Game developers in the USA. Austin is also pretty huge but nothing beats the Bay area. Of course all of these areas pale in comparison to Montreal especially when you consider that being in Montreal means that you get up to 35% of your labour costs refunded to you at the end of the year and in terms of game dev's per capita it's even higher than California.
It's an amazing feeling to be running into game devs everywhere you go in this city, to have game posters be proudly displayed alongside movie posters at government buildings at demonstrations of the city's accomplishments for the year. The game dev culture here is pervading and everywhere you go, I wouldn't recommend any other city besides montreal to a development looking to setup shop here.