Gnome

Gnomes need an unsafe wall behind them. Plain dirt and grass walls near the surface line work, and living wood walls work far better at any depth, which is why the root tunnels of a living tree are thick with them. Living wood also loosens the rules elsewhere: they start turning up against other walls, and after dark they can appear with no wall at all. Corruption and crimson ground shuts the dirt and grass route down completely, and a blood moon or an eclipse shuts everything down except the living wood one. Three can exist at once and each one alive cuts the odds of the next, though living wood spawns ignore that limit.

Remix and Zenith drop the living wood rules entirely. Dirt and grass walls become the only way in, at twice the usual rate, and the search moves to the cavern layer above the mushroom zone.

A gnome caught on the surface in daylight with no background wall behind it freezes into a Garden Gnome, and a placed one raises luck for anyone standing near it. That is the entire reason to care about the species, since they carry no drops of their own. The change still counts as a kill for the banner and the bestiary, credited to whoever is closest, so the ornament costs you nothing.

Watch for the white glitter: conditions are met and it is only waiting for a clear block to stand on. It flattens wild grass to make space but cannot shift placed furniture, so a group petrifies one at a time given room. In Expert worlds, any coins it has picked up are gone for good once it turns.

The problem is not finding them, it is the climb. Their knockback is heavy for that stage of the game and a living tree interior is a vertical shaft, so every hit throws you back down the hole you are climbing. A Cobalt Shield or anything built on top of it removes the issue outright. Failing that, line the shaft with platforms or cut a staircase, and remember they clear six tiles in a jump, so one platform is not a barrier.

Build a long flat run of blocks and cap the strip with overgrown living wood wall, then stand at one end outside the walled section. They spawn along the run, walk toward you, leave the wall behind them and petrify where they stand. Before the aether exists and luck matters at all, the cheap version is a Battle Potion and Water Candle inside a natural living tree.