With recent versions of gMail you cannot turn off the mail notification sound because they've removed that feature from within the gMail options. You can, however, place the Power widget on one of your screens and then disable Sync when you go to sleep. This way gMail will not be checked. and you can still get important phone calls or text messages.
I'm rooted, so I run gMail version 2.3.2 which has an option to use a specific notification sound, and Titanium Backup to prevent it from being updated by the Market. Why they removed this basic function in more recent versions that clearly a lot of people need is beyond me.