first tools needed is background check
use one that allow email lookup
ie :
steps* search the email on pipl.com or on advance background check
to know the state and country of the email owner.
once you get the state and country use sock of that state or RDP
( i prefer RDP, because when you use the email Country RDP at times state dosent matter).
after that login to the gmail account, it would either ask for confirmation of recovery email
or phone number and it will dsiplay part of it.
thats it Simple x)
Note: to get the recovery email and password. look up the email account on background check.
you will see the other emails adress and phone numbers.