There are many reason of Gmail app not working on mobile and computer:
1. First you go the gmail account setting and enable the IMAP if you get IMAP turn off then you would not be able to get email from the server.
2. Remove your gmail account from your iphone, android and set it up again your gmail account.
3. Fix the SMPT error
4. Still you did not find any solution then contact gmail customer help assistant
