When Gmail won't connect to the outlook 365 app. it means, something is wrong with user account or password.
1. Check the username and password for Gmail account.
2. Go to the imap and pop3 server settings.
3. Remove the current account and then re-add the mail account.
4. Configure the Gmail account on the mail application.
Once you will make these changes into the Gmail account. your account will start working fine.