Let’s assume that:
- you want to chat with me;
- you only have my publicly available email address (`email@example.com`);
- you suspect that I also have a gmail account linked to the `firstname.lastname@example.org` address;
- you want to find it so that you can initiate a chat.
It is easy. Create a new Google Site, and enter the Site settings/Sharing tab. Add my name as a viewer and click Invite these people.
Then select Skip sending invitation so that I am not notified.
email@example.com email address was associated with my gmail account: you got it, you can start chatting even if I never intended you to get access to my gmail address. Congratulations!