Once the gnomes are decorated it is time to bake them. Sculpey is an oven-bake polymer clay. So it is soft and plyable until it's baked. First, heat the oven to 275 degrees F. While waiting for the oven to pre-heat put some foil on a cookie baking sheet. Then carefully put the gnomes on the cookie sheet being very careful not to let the Sculpey touch the sheet or the rack. Put the entire gnome in the oven and bake for 15 minutes. Put the bat-a-rang on the sheet too. Since the gnome is ceramic it's safe to put it in the oven. The kiln that it was fired in was a lot hotter than your kitchen oven. When the 15 minutes is up take them out and let them cool completely to room temperature. Note: if your Sculpey pieces are thicker than about 6mm then you can bake it a few minutes longer than 15 min.
Remember, there is nothing holding the Sculpey onto the gnome (no glue or anything). But the Sculpey will probably stick to the ceramic really tightly anyway. I doubt that the Sculpey will stick to the gnome long term so you should carefully pry the Sculpey piece off and glueit back on with construction adhesive. This will hold pretty well outside.