Did you try the following steps correctly?
- From the Home screen, tap menu > Settings > Accounts & sync.
- Tap the Auto sync On/Off switch to turn automatic sync of all your accounts on or off.
- To manually sync individual accounts, tap an account type, and then do one of the following:
• Tap on three horizontal dots next to the account you want to sync, and then tap Sync now.
• Tap three vertical dots > Sync now on the Account settings screen.
If this doesn’t help then try these steps. Hope it may help you
1. Remove Your Gmail Account and Re-add it.
2. Clear Cache
3. Check Sync Settings on your Gmail App
4. Factory Data Reset