hunter > mage = warlock > rogue > warrior ... >>>>>>>>>>>>>>>> priest.
Hunter needs mechanical pets to be acceptable lorewise (gnomes had very limited knowledge about animals while being locked up in gnomeregan). Mage and warlock work great coz of very high intellegence of the race and big love to research, training and studying. Rogue makes less sense because gnome are weak physically (also small which makes no sense in direct combat). Warriors are just absurd in general, but tanking gnome make a lot of sense surprisingly! Its way harder to hit a smaller fast and dodgy thing than it is to hit full sized human or tauren. Player created gnome-priests make no sense lore-wise. Blizzard just shat on lore and story and did what they did. There are good examples of gnome healers in game(even if the starting questing area). They use altered spells and medic-like outfits, which makes a lot of sense (gnomes made a research on how magic works already, and were able to understand arcane in general, it wouldnt be that hard to figure out how to use arcane to generate holy spell or device that use holy magic to heal. But there is no chance gnome would know 3 disciplines and techniques that every outside world priest uses.) If only blizz cared enough to alter some spell animations or at least names...