My niche for Gnome ? Hey, what Gnomes are supposed to be: keeper of knowledge. They are overlooked and ignored by the fools, and this allows them to go everywhere they want, and to record everything they want, unnoticed. Gnomes knows. Their very name comes from knowledge (gnosis, in greek, which means knowledge or wisdom).
Forest Gnomes are keepers of nature's secrets. They are instrinsically, innately much more attuned to nature than a sylvan elf druid.
Deep Gnomes are keepers of science's secrets. They are the only "tinker" gnomes I have, and they are more "engineer" than "tinker", if you see what I mean. They don't design useless gadget that blows them up, but efficient machinery for their underground lives, like magically-motionned subway trains, watermills, digging machines...
The mysterious Night Gnomes are the keepers of cosmological secrets. They knows the old legends and prophecy. They know the politics of gods. They know what goes in the other planes. And they are themselves a legend, tiny ghostly silhouettes seen from a long range, performing complex and beautiful rituals -- but if you come closer, you discover it was only a mirage. The lake you've seen, with all the gnomes around, don't exist. There's nothing here. You turn around, and promise yourself to drink less beer from now on.
Finally, rock gnomes are merchants and travellers that serve as link between the other gnome subraces, and between the gnomes as a whole and the other races.
Deep gnomes are known to dwarves, who enjoy working with them, and forest gnomes are known as the "weird people that live in the heart of the forest, where you can't go because of all these big angry giant foxes, pit concealed by dead leaves, thorny bushes, and disturbing acoustic phenomenons" to some druids. They are druid themselves. But other than that, they are little known.
In fact, gnomes are the Illuminati: a secret, invisible hand, which control (a little) the world through its knowledge and its secrecy.
Ain't that a perfectly good niche ?