Taken from the official game guide:
" * Treasure Gnome
- Souls, Gold, Glory, Keys, Diamonds, Vault Keys, Traitstones
-
Pet Gnomes
- Trigger a 1 hour Pet Rescue for your Guild
- Glory Gnome
- Gives more Glory than Treasure Gnomes
- Soul Gnome
- Drops Souls or Vault Keys with a higher chance of earning a Vault Key than a Pet Gnome
- Jewel Gnome
- Drops Jewels, Diamonds, Shards or a Vault Key
These are in approximate order of rarity."
From the last sentence I presume this is intended to mean we will see Treasure Gnomes the most often on average, then Pet Gnomes etc. I wonder if this is actually the case though? 