By continuing, you authorize a one-time, read-only copy of this entire Gmail account — all messages and attachments — to be saved for business and record-keeping purposes. The tool cannot send, delete, or change anything in the account, and you can remove its access at any time in your Google Account settings.
You'll then see a Google screen saying "Google hasn't verified this app." That's expected for a private tool — tap Advanced, then Continue, then Allow.