(Note, I am not a superadmin in my domain, so I do not have access to these settings.)
It used to be that to allow access to a Google for Education domain it could only be to another GFE domain and both domains had to mutually allowlist each other. "Gmail.com" address could not be allowlisted. Another admin (not a member here) said that his domain could be opened to external users without mutual allowlisting. Has something changed?
Mark Loundy (He, Him, His)
Instructional Technology Specialist
De Vargas Elementary School
Ignited Fellow
Google Certified Educator