Cataclysm extended the availability of many classes to races that previously hadn't had them. We got some common-sense ones like Troll Warlocks or Blood Elf Warriors, and some that reflected new lore, like Dwarf Shamans and Tauren Paladins.
Gnomes did get Priests, their first healing class, thanks to the notion that Gnomes would experiment with healing magic to serve as medics. But there was a giant missing piece:
Gnomes are known for their engineering - they have a passive that raises the profession skill if they choose to go that way. Engineering has always been able to build guns, but the sad fact is that those firearms, so lovingly made by Gnomish hands, could never find use among their creators (except as stat-sticks for Warriors and Rogues before Mists got rid of the ranged slot.)
It has been a huge oversight - until now.
Yes, while there might not be the huge race/class combination expansion we saw in Cataclysm, Legion will finally be allowing Gnomes to shoot their foes from afar and befriend animal companions.
One kind of interesting thing is that this means that Hunters will join Warriors as universal classes, available to all races. The only other class to ever have that status was the Death Knight, though the introduction of Pandaren broke that status for them. Warriors went for two expansions without being available to all races, but Cataclysm gave them to Blood Elves.
Anyway, Gnome Hunters incoming!