There are different types of gnomes and each expansion pack comes with a handful of additions. Here they are and how to get them
Base Game (Vanilla) Mysterious Mr. Gnome - the usual gnome you'll get while fishing, mining, exploring catacombs. Getting them instead of a gem is a bit hard, as you can only do this after you have one and had knocked that gnome before, so this is some kind of revenge
World Adventures expansion pack 1.
Sultan Sam (Al Simhara, Egypt) - occasionally found while cutting gems in Al Simhara or exploring the local catacombs, sometimes found through fishing.
2.
Master Manchu (Shang Simla, China) - all the same as the latter.
3.
Little Leon (Champs Les Sims, France) - all the same.
Ambitions expansion pack 1.
Abracadabra (Laundry gnome) - suddenly found while doing the laundry.
2.
Sculptor's gnome - found when sculpting.
3.
Magical invention gnome - found when inventing.
4.
Trogleite - found when using the Time Machine (big invention).
Late Night expansion pack (sorry can't say how to find them, not much info about them, but once I get LN, I'll update this)
1.
Hip-Hop gnome (male pig) 2.
Vampire gnome (female vampire)Here's a pic I found while surfing the net.