Github should donate a license for Github Enterprise to gnome so that the gnome folks can have the best of both worlds - self hosted + github. Its a win for github to have a big project like gnome running on GHE.
Gnome already discounted Gitlab Enterprise as an option because its proprietary.
It is very hypocritical to promote free software while using proprietary software that you got special privilege for on the backend. KDE had the same line of thought when they chose Phabricator - if features like LDAP weren't locked behind Gitlab EE they would have used that, but they were only considering freedom respecting options, because they are communities of hackers that want access to improve their tools.
What kind of LDAP functionality did KDE want? LDAP login is in GitLab CE (open source) but LDAP group sync is in GitLab Enterprise Edition (proprietary).
GNOME is part of the GNU project, so the only "pressure" they could apply to GNOME is to make their enterprise offering entirely free software (at which point they will run it themselves because of GNUs and the FSFs position on Service-as-a-Software-Substitute[1]).
It is very hypocritical to promote free software while using proprietary software that you got special privilege for on the backend. KDE had the same line of thought when they chose Phabricator - if features like LDAP weren't locked behind Gitlab EE they would have used that, but they were only considering freedom respecting options, because they are communities of hackers that want access to improve their tools.