GNOME is part of the GNU project. If you think that the GNU project would use a proprietary product, you might want to have a refresh of what the GNU project stands for.

The fact that Linus used proprietary software to manage Linux was not seen positively by the community and eventually (after one of the developers tried to write a free client to interact with their proprietary service) everyone in the free software world got a reminder why we shouldn't promote or use proprietary services (the "gratis tier" access got revoked for the project).

GNOME isn't really part of GNU these days.

That is a pretty terrible example to give considering how it turned out.