STEP ONE: Make Sure You Have The Latest Version Of The Gmail App
If your main trouble is sending or receiving Gmail then the problem is likely an issue with what version of Gmail you're running on your device. You can download the latest Gmail update for Android here:
Gmail - Latest Android App For Google Play
STEP TWO: Restart Your Device
Restarting your phone or tablet will often solve weird in app bugs. A general good rule of thumb is to always try the simplest fix before you get too deep down any rabbit holes looking for an answer!
STEP THREE: Check Your Android Phone Settings
Check if you're online
Turn off airplane mode
Check your Gmail sync settings
Check your Android phone or tablet settings
STEP FOUR: Clear Storage Space On Your Phone
For instance, if you're having trouble syncing your Gmail, it could be that you're running out of storage space on your Android phone.
If your phone or tablet is running out of space, here's how you can make more space:
Uninstall apps that you don't use
Delete downloaded files, files you created that you don't need anymore, or move those files to your computer
Remove Google Play music, books, movies, and TV shows you've downloaded
STEP FIVE: Check Your Password Is Correct
On your computer, open Gmail. If you can't sign in or get one of the errors messages that say: "Username and password not accepted," "invalid credentials," you're asked to enter your username and password over and over...
Here's What You Can Do:
Make sure you enter your username and password correctly.
If you use 2-Step Verification, you might need to sign in using an app password instead of your regular password.
If you still can't sign in, follow these troubleshooting steps.
STEP SIX: Clear Your Gmail Information
WARNING By following these next steps there is a chance that you might erase your drafts, your Gmail signature, ringtone, and other settings.
NOTE You should only try these steps if the above steps didn't work.
Open your device's settings.
Tap Apps > Gmail.
Tap Clear data > OK.
Restart your device.