If it is this wiki:
https://sims.fandom.com/wiki/Mysterious_Mr._Gnome
it does not always list all information. For example a summer gnome, Brawn Brawn G'nome, can be had just by it being summer. For me that one pops up whenever during summer, even in saves where the sims do not have pool parties, makes BBQ, or keeps fireworks on their lot; those just being extra weight on the chance of getting one.
The "HoudiniGnome" can be created by a magician at level seven and up, doing the Appearing Objects trick - maybe the household had a wandering wizard visiting :grin:
There is an old and in terms of the game's age early made, now outdated, modified "Garden Gnome", that makes plants grow fast; that perhaps could account for some confusion
http://modthesims.info/d/393965
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