They're just regular magic gnomes, much like the vampire and teddy bear ones, only appearing if you meet certain requirements. The aging gnome is from Generations, I think. I didn't know I had one until it died and a golden statue took it's place. The golden statue still moved around like the gnome.
The only gnomes I remember to have an actual function are the pet-prohibiting ones (3 gnomes that hold signs, forbidding strays of each species depending on their signs).