I've always said gnome like gnu. But i'm not sure about some other apps that start with g like some of the things written for gnome.
From the gnome faq:
Q: GNOME: "nome" or "guh-nome"?
A: In English, the word "gnome" is pronounced with a silent "g". However, GNOME itself is generally pronounced as "guh-nome", just as GNU is pronounced "guh-noo" when referring to the GNU Project.