The recovery of a GMail which was compromised is something you need to have taken up with the GMail forum folk as we are not as versed on matters GMail. If you had had a YouTube channel which was hacked you would have been advised to use the compromised account form here but that has now been retired.
Unfortunately there is no way you can verify ownership of a legacy YouTube channel unless it was linked to a Google account and you knew the Google account (ie email) it now uses. And given that it may or may not be linked and it may or may not still have the GMail on its record as the email you really don't have many options here.
You could flag your own videos if they infringe your rights...?