possibly the app got stuck trying to pull in fresh data - if you haven't already, go to settings>general>apps, find gmail and tap on it. Force stop, clear app cache, and reboot. I find a spike in battery use is generally due to a "stuck" app - and force stop/clear cache/reboot will usually bring things back to normal. Although I find Google Play Services tends to be the most common one I see people asking about, I assume it's possible for other apps to get stuck as well