IBM bought Redhat, Redhat hires the devs for GNOME and hosts its infrastructure.
IBM and Redhat where also the main founders of GNOME.
They don't need to legally own GNOME. When most of the people working on systemD and GNOME are IBM employees, IBM makes the decisions. And GNOME is an IBM company.
https://wiki.gnome.org/RedHat https://en.wikipedia.org/wiki/GNOME_Foundation