How much of your old account do you remember?
It looks like the process is to login with your gmail and then link it to your old account. If you remember the username/email, that's a good first step. But if you don't remember the password, that could be tricky, since they might send a "recover password" email to that account and if don't have access to it anymore, that's bad. But Google/YouTube usually has contingencies for that.
So, step 1... log into Gmail. Step 2... link it to your old account.