It depends on your view of history.

The G comes from the GNU Project. While GNU is an acronym ( GNU is Not UNIX ), the accepted pronunciation is a hard G ( GUH-noo ).

When the GNOME project was started ( and named ), it too was an acronym where the G was GNU. So, it seems very reasonable to use a hard G.

GNOME is no longer affiliated with GNU and the project has stated that it is no longer an acronym although it is still capitalized. If the G is not GNU, it makes total sense to pronounce it as the mythical creature of the same name which is pronounced as a soft G.

I have not seen anything official on how to say it from the project itself. So, it may be a matter of personal preference at this point.

I use a hard G because that certainly WAS the proper name and I have not seen anything official saying they wanted to change it. They have kept the capitalization.