It depends if the account has been linked to a google account or not. If it has been linked to a google account, then you can use this:
If you don't have a google account attached to it, then you'll need to link it with this gaia link, but this requires you to know your login info for that account:
This link states if you don't remember your old password, and if it hasn't been linked to a google account, then there isn't a recovery option... :/
This seems to be different than how I remember the process in ~2015. I could have sworn it used to be recoverable if you could remember the email address but forgot the password, but not recoverable if you had forgotten the email address. Hmm....
Hopefully you had created a google account, because then I think you could follow the first link to recover the password...
On the google forums, someone mentioned trying to create a google account with the same email as the legacy YouTube channel, and then linking the channel with the gaia link, but I have no idea how that would really work..so.. :/
Here's a thorough post on the google forums that would be good to read through, too!