If this error is occurring on mobile devices, the problem can be related to your internet connection. The best way to fix Gmail not loading issues is to reset the Gmail app's settings. This will not delete the data on your mobile device, but will reset your phone's settings to their default values. If the problem persists, you can contact your internet service provider for assistance. Then, you can try resetting your Gmail on mobile.

See also - https://apfashiontech.com/how-to-solve-gmail-not-working-issues/