1) In Android there is no way (circa 2015) to give access to some google apps but not gmail (you can uninstall, but nothing preventing a reinstall)
2) So… some recommend AppLock or some other applock app. Add a passcode to gmail and you are set. As long as it’s impossible to uninstall the app lock app itself!
3) But… all of the one’s I’ve looked at give themselves rather excessive permissions. Funny! So forget it. http://android.stackexchange.com/questions/18747/why-do-app-locker-apps-need-so-many-permissions-am-i-safe
I mean, it’s commonplace for Android games to do the same thing, but sheesh… for an app lock app itself?!?! Crazy!