Sorry, if you forgot and OBI "forget", how would you expect any of us to know what it is?
Usually when you set up any gmail account you are asked for a recovery email. Once entered, an email is sent to that recovery address. If you don't have that many emails, do a search within those other possible recovery emails for sender: accounts.google.com. Hopefully, it will contain the info you need.
If you've never setup a recovery email you may be SOL.