The simplest method, and the one most clients use: forward an email to your agent's address manually, exactly as you would to a colleague. No OAuth authorization, no API key, no access granted to your Gmail account — the agent only ever sees the messages you actually send it.
To automate this, open Settings > See all settings > Forwarding and POP/IMAP in Gmail, add your agent's address as a forwarding address, confirm the verification code it receives, then choose to forward all incoming mail automatically or only messages matching a filter (sender, keyword, attachment). This entire step happens inside your Google account — no login information is ever shared with us.
