See
Troubleshooting Database Mail[
^] and inspect the mail status as suggested there to know what happened. If you are still not able to fix the problem yourself, add the reported errors to your question (use the green 'Improve question' link).
Ensure also that you met the requirements for sending to GMail accounts (server:
smtp.gmail.com, port 587, check box 'This server requires a secure connection (SSL)' checked, valid user name and password) and that you have allowed less secure apps to send mail (see
Allow less secure apps to access accounts - G Suite Administrator Help[
^]).