Bob, try this link:
https://myaccount.google.com/device-activity
It should give all devices's activity on Google. For instance if you access gmail through Google's Inbox, it won't show up in Gmail's activity.
Also, double check all of the apps and their permissions, here: https://myaccount.google.com/permissions Check if they have permission to access your email, and if so, ask yourself if it could be that such app followed the reset link in a legitimate way.
If you find nothing, follow the instructions on
I think someone else has access to my account
on https://support.google.com/mail/answer/45938?hl=en
I won't post the detailed steps as those might change with the service, but hopefully the link to that answer and page will be updated by Google
(but as they said, if they used your computer, then that's not good, or if Paxful got compromised)
Also, check if the Paxful email is a phishing one. It could be that the links sent you to a similar webpage / or one on Paxful that got HTML injected to it, to steal your Paxful credentials.
As you said on the comments, 2FA (/MFA), for the win. But make sure you have those 2FA keys on more than one physically secure site, something like your wallet and your place at home to keep stuff safe should suffice.