Depends on the setup, but programmatic access to a Gmail account that's used for admin purposes would allow for hijacking via key/password exfiltration of anything in the mailbox, sending unattended approvals, and autonomous conversations with third parties that aren't on the lookout for impersonation. In the average case, the address book would probably get scraped and the account would be used to blast spam to the rest of the internet.
Moving further, if the OAuth Token confers access to the rest of a user's Google suite, any information in Drive can be compromised. If the token has broader access to a Google Workspace account, there's room for inspecting, modifying, and destroying important information belonging to multiple users. If it's got admin privileges, a third party can start making changes to the org's configuration at large, sending spam from the domain to tank its reputation while earning a quick buck, or engage in phishing on internal users.
The next step would be racking up bills in Google's Cloud, but that's hopefully locked behind a different token. All the same, a bit of lateral movement goes a long way ;)
