Google has recently changed what permissions they allow users to give applications, and it works differently for normal Gmail accounts versus G Suite email account (e.g. work accounts).
If you have a G Suite work email account, you can ask your G Suite admin to white list the WhoKnows app, so it'll show as verified. Here are the instructions you can send your G Suite admin to help them.
If you have a normal Gmail account, Google doesn't ever allow any application that analyzes professional relationships via email activity to be white listed for Gmail accounts. You can still import your Gmail by following these instructions:
1) When you see this screen after clicking Connect in your WhoKnows settings, click on the Advanced link highlighted below.
2) Then click the "Go to WhoKnows" link highlighted below. It is safe as WhoKnows only analyzes the email headers (e.g. to, from, and cc addresses), no message content or subject lines.
3) Afterwords, you'll see the normal permission screen granting limited access to WhoKnows like this.