Every solution starts with restarting your device then try this universal solution to fix Gmail not syncing issue. If the solution is not working for you then follow these troubleshooting steps -
1. Check out if your IMAP Setting is Enabled
2. Reinstall your Gmail App
3. Check your Internet Connectivity
4. Check Push Settings
5. Remove and Re-add your Gmail Account
See also - https://emailspedia.hpage.com/why-is-gmail-not-syncing-on-iphone.html