You need two console commands

1. First you need to create a gnome model with this command
   
   sv_cheats 1; prop_physics_create props_junk/gnome.mdl; sv_cheats 0
   
   
2. Next you need to give the name "gnome" to this model
   
   sv_cheats 1; ent_setname gnome; sv_cheats 0

Now all you have to do is throw the gnome into the core and get the achievement