Go to the login and initiate the password reset and follow the prompts

Google offers numerous methods to recover your account IF you've set them up ahead of time.

including backup codes - if you've set up 2 factor authentication

Recovery to multiple alternative email addresses and phone numbers- but you can't do this retroactively in other words unless you CAN login

You can obtain a replacement sim if you lost your phone and het it assigned to the other number- you could even maybe borrow a friends phone and obtain the sim from YOUR provider

You can use alternate 2fa devices like 

You might be able to respond to security questions like when was the account created

If everything else fails google say make a new account- anyone claiming to be able to recover the account for you is ripping you off there are no back doors.