Depending on what precisely they do with the data, they could do things quite a bit more damaging than turning off your Gmail account, though it certainly wouldn't rise to the level of sending you to gitmo. The worst would probably be some kind of financial and/or career blackballing, if they sold data-mined information about you to credit agencies, lenders, private investigators, and/or prospective employers, and something particularly damaging were contained in it.
I believe their privacy policy currently says they won't do that, and it's especially clear that they won't do anything except target ads using information mined out of your email. But I don't think there would be anything illegal about it, as long as they didn't do something like outright blackmail (offer to let you pay to expunge negative information or something).
The difference is that if Google did that, everyone would immediately stop giving them that information, which is worth more to them than the advantage they could gain in screwing you over once.
On the other hand, if the government does this, people can do... what, exactly? Stop giving their information to the government? They didn't willingly do that to begin with. Stop giving it to Google? Maybe, but they would have to stop giving it to anyone, which is a lot more difficult than just switching from one service to another.
I believe their privacy policy currently says they won't do that, and it's especially clear that they won't do anything except target ads using information mined out of your email. But I don't think there would be anything illegal about it, as long as they didn't do something like outright blackmail (offer to let you pay to expunge negative information or something).