He's not, but he wants to be.

Ipadidymus was not a gnome, but then again he wasn't a human, a dwarf, an elf or even a pixie. That's not to say that he didn't want to be a gnome. It's not even that he didn't try to be a gnome. He worked very hard to fit in. He was oh so friendly and helpful and he was loveable. Ipadidymus just wasn't a gnome and wanting to be one as badly as he did wasn't enough to make it happen. He knew that, but he refused to stop trying. After all, Ipadidymus loved the gnomes and knew that being one would make him happy.

Gnome Law

Rule Number 1 of Gnome Law is that all gnomes are required to have parents that are gnomes. It appears to be a reasonable law, one nearly everyone could understand, but here's the bend in the road for Ipadidymus. He was orphaned before he was old enough to cling onto memories, and all he ever knew was the gnomes, and Norge Hollow. It made sense to him that he had to be a gnome. He tried very hard to be one, but being so large, so different in appearance and obviously not of gnome parents, the first rule of Gnome Law left him to be who he was. Sadly, he was not a gnome.