I had the same problem with my Samsung Galaxy S5 & S6. It's in the SYNC settings within Gmail.
Go to Gmail app and go to "Settings".
Select your email address, below "General Settings".
Scroll down to "Data Usage" section.
Make sure "Sync Gmail" is checked.
Select "Days of Mail to sync" and change to "30 days"
Select "Manage Labels"
Select "All Mail" and then select "Sync Messages"
Change to "Sync: None" (I know this sounds weird but it works).
If you have a lot of folders, like myself, there are only certain folders that should be synced:
- Inbox - Sync: Last 30 days
- Priority Inbox - Sync: Last 30 days
- Sent - Sent: Sync All
Then close out the app, and reopen. This has worked for me and several people I know that are on different devices. Good Luck!!!!