The most common cause of Gmail sync issues is between the main account and the Android app.
Follow these steps to sync Gmail error on your Android device:
Perform a manual sync. Open the Gmail app and swipe down from the top of the screen.
Enable automatic syncing. If you prefer not to sync manually, you can automate this in Settings. Open the Gmail application, touch Menu (the three-bar icon), then touch Settings. Click on your account name. In the Data usage section, check the Synchronize Gmail checkbox.
Make sure the device is connected to the internet. Check that the device is connected to Wi-Fi or that mobile data is turned on to sync the Gmail app.
Check your password: go to mail.google.com and sign in. If you get a password error, this may be the reason why the app is not syncing properly. Change your password on both devices.
App update: Downloading the latest version of the Gmail app can fix Gmail syncing issues. Visit Play Store; If you see Update next to Gmail, tap on it. If you see Open, then you are running the latest version.
Delete the Gmail application data and stored Gmail files. Cleaning up storage space can fix technical problems with applications.
Open the Settings app. Touch Apps & notifications, or touch Apps in older versions of Android. Touch the Gmail application. Tap Storage & Cache> Clear Storage and then confirm the action.
Restart your Android device. Sometimes a reboot is all it takes to get your smartphone or tablet working properly.
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