@Anmira So this gnome's appearance has nothing to do with the fact that my sim has a garden? Most of my sims do garden, and the gnomes tend to congregate in and around the gardens, and even move there en masse while my sim is tending to the garden. But I've never yet played a magician or had a magician gnome before this. So I guess if it really was the garden that caused him to appear there, I should have seen one before this.
I did sort of test the theory of the magician gnome having an effect on the garden plants, by placing him next to the plants my sim is growing indoors in pots during the winter, but they're not growing any faster.