You can display the gnomes anywhere in your yard, but it is best to place them in an area where they can get a good view of the entire yard and any particular trouble areas. If you've modified your gnomes correctly, your garden should be in order within a few weeks. Any new threats can be handled with additional super gnomes or by forming a super gnome league or justice guild.
Take your gnomes inside during winter. The crime rate drops dramatically during the colder months anyway.