How to sync all emails for all labels without individually going to each label and selecting sync all? I switched from the native email app to the Gmail app because I could finally have separate views for my Starred folder. But now I'm faced with another issue: I can't seem to find a way to sync all emails for all labels without individually going to each label and selecting sync all? Is this possible?
That shouldn't be a problem at all!
Go to settings>accounts>Google>tap on Gmail (to untick the sync), and tap on it again to sync right away. 
Alternatively, you can also tap on 'Sync Now' option, which will sync all your Google apps.

Note: make sure you don't have sync only on WiFi option enabled, if you do, either be on a WiFi network, or disable the WiFi only option! :)