My company uses a Gmail-based email server. For integration with a partner's system I have been asked for my Gmail "user ID", here is what they wrote: there are usually two emails associated, one should be firstname.lastname@example.org, the other one should be userID@mycorp.jp. Searching the web reveals this question on StackOverflow:
How to recover a Google/Gmail username - YouTube May 01, 2013 What to Do When Forgot Gmail Username Dec 18, 2018
How to Recover Your Forgotten Gmail Password
Gmail Availability Checker is a free tool to check if a Google Mail username is available. It instantly checks to let you know if a username is available as you type. It allows you to quickly and easily find the best Gmail email address.
Aug 03, 2011
May 01, 2013 What to Do When Forgot Gmail Username Dec 18, 2018