I forgot my Gmail password so I can’t get on to my Apple ID

If you have forgotten the email account password, not your Apple ID password, try going to the Gmail web site to look for a reset option.

Gmail Support.     

See If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

You also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid 

And you undoubtedly will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485