Google may block sign-in attempts from some apps or devices for third party services like Reply. If Reply shows you an error message when connecting it to Gmail, try these two methods to make things right:
1. In Google Mail settings, go to Allow less secure apps and choose “Allow” to let less secure apps access your Google account.
2. If testing shows an error only when checking your IMAP settings, go to Settings -> Forwarding and the POP/IMAP tab -> Enable IMAP.
3. If you're getting an 'invalid credentials' error, make sure your password is correct. If you’re sure your password is correct, you can try these troubleshooting tips: My client isn't accepting my username and password.
This will allow you to both allow Reply to connect to your Gmail and make sure your account settings are correct.