They aren't porting it to GNOME, per se, but to GTK. GTK is a popular GUI toolkit on many Unix operating systems. GNOME is based on GTK, but GTK really got its start with the GIMP (GTK actually stands for "GIMP Tool Kit"). Almost all GNOME apps are GTK apps, but many GTK apps are not GNOME apps; GIMP isn't a GNOME app, for example.
GTK is also gaining popularity in embedded applications, and this may be why Nokia decided to work with it. Mozilla and Minimo (its embedded counterpart) are very good, but they carry a lot of baggage with them which isn't necessarily optimal for embedded stuff. GNOME isn't really optimal for embedded apps either, though there is talk of making a GNOME app around WebCore-GTK.