I was a Gnome engineer once but respecced mining/engineer to inscription/tailor, but now am going back to mining/engineer. While I always had the gnome engineer icon under the general spells, it is gone since patch 3.0.2 so it seems I am able to specialize again.
While I prefered gnome engineer for all the trinkets in vanilla and bc, the only thing that only gnome engineers can wear in WOTLK I've seen are the goggles that make everybody naked. Which I really, really don't need. I haven't found anything that can be used by only goblin engineers though.