While it's not a commonly used word, it's a valid way to describe the qualities associated with gnomes, such as:
* Their mischievous nature: The gnomishness of their pranks was legendary.
* Their connection to nature: The gnomishness of the forest was palpable.
* Their small size and stature: He was impressed by the gnomishness of the tiny creatures.
However, it's important to note that "gnomishness" is a rather obscure word. In most cases, you could use more common words like "gnomery" (the actions of a gnome) or "gnome-like qualities" to convey the same meaning.