X1 Search does not yet support Gmail 2-Step verification but this is certainly on our roadmap to address in the future.
In the meantime, Google allows for application-specific passwords to deal with products like ours that don't yet have the UI to handle the 2-step process. This link explains how to create an application-specific password:
If two-step authentication isn't the issue and you're still unable to add Gmail as a data source, there is another potential solution that may work for you:
Log into the Gmail account(s) and go to this link: https://www.google.com/settings/security
Gmail has recently created a setting under Sign-in & Security > Connected Apps & Sites for "Allow less secure apps." If this option is disabled, then X1 Search will be unable to connect to the account.
If disabled, toggle the switch for this setting to ON and authenticate in X1 Search again.
If all else fails, you may also need to enable IMAP in your Gmail settings. Please review this article from Google on how to check this: https://support.google.com/mail/troubleshooter/1668960?hl=en#ts=1665018