Had a look on the guide, couldn't find anything on this. I'm trying to get 1 of every gnome without buydebug in one of my games, so am trying to compile methods to get each one, and get a full list of all the different gnomes. Below is a list of all the gnomes I've got before, and what I think the various ways of getting them are.
Mysterious Mr Gnome - Transfiguration, sending Metal to get smelted, Mausoleum, Miner
Sultan Sam - Exploring Tombs in Egypt
Little Leon - Exploring Tombs in France
Master Manchu - Exploring Tombs in China
Inventing Gnome - Random chance while Inventing, Miner
Abracadabra - Washing Machine - only the most expensive Ambitions one
Vampire Gnome - Plasma Fruit growing on lot
Bucktooth Butternut - Egg hunt
Troglodyte - Travelling back in time
There's also a load other gnomes in the buydebug that I've never seen before and am unsure on how to get them:
Mr Gnome's Generations Family - 2 Mysterious Mr Gnomes have a family - gnomes go nooboo to elder then die to get a Memorial to a Magic Gnome
Blingaboo - Gained as a rare gift at level 5 Celebrity
Brawn Brawn G'nome - Summer Festival / Spending time outside on the lot / Possibly Tanning
Carter Caninimus - Becoming BFFs with dogs in the house
Cracklin' Nackerbell - Fall Festival and Haunted House
Cranstan Boonitz - Magician Career?
Evil Mr Gnome - Similar to Mysterious Mister Gnome for obtaining one, also from just being Evil
Felix Felinimus - Cats playing with the Catnip Mouse
Freezer Bunny - When a dog barks at a freezer
Gnome of the Darned - Obtained via dying from the haunting curse
Lounge Lizard - When a Singer gives an epic performance
Sculpting Gnome - Random chance while sculpting
Zenyatta - Small chance when a horse licks a salt lick
s-gnome-man - Winter thing
And apparently the Consignment Store and the Claw Machine are good for getting a large number of different gnomes.