I have had ghost gnomes just turn up. Or ghost gnomes seem to appear if a Sim in the household dies and their tombstone is on the lot. See several posts of mine with pictures on page 2 of this thread. I might also add the grim reaper snowmen were built when a Sim died and her tombstone was on the lot. So you seem to get ghost gnomes and grim reaper snowmen if you have a tombstone with a ghost in it on the lot.
When there is a ghost gnome in the household lots of ghosts tend to show up and do things around the house - including picking up a toddler and sitting in a rocking chair and rocking it.
Here is a picture I got of the decor objects that look like gnomes in Sims 4.
http://s5.postimg.cc/yyj4ith1j/Gnomes_Sims4.jpg
So far I haven't found anything in Sims 4 that is like the gnomes in Sims 3. But I haven't been spending much time playing Sims 4.