Gnome and Gtk+ were not fit for phones: not prepared for small screen sizes, lack of adequate widgets, slow, difficult to develop for (types in Gtk+ were and still are a joke), etc. And first and foremost there was the attitude of those Gnome developers: entitled, reinventing the wheel all the time, arguing stupid arguments on purity... To this day, Gnome and Gtk+ are not fit or mobile (or Windows or Mac) yet.
That sounds a bit like a Gtk+ vs QT mud throwing contest.
The N900 had pretty good reviews and 2 more years of work could likely have made it even better. And Nokia had the developers and control to do what they wanted however arogant Gnome developers were.
I have developed for neither, so I can't really say.
That said, I have been using Gnome based desktop for 10 years now, so I'm bias towards it.
And what the problems were with gnome? In 2009, the N900 was based on GTK and was a solid base to build on.