to do this you have to open the game

when you are in game click on play campaign and then click dark carnival

first you do not appear in the carnival you appear in the street you have to first go into the hotel and then you go through a a lake or something like that

in the attractions there is a shooting game (the attractions are when you go into the safehouse) you will find a shooting attraction called the fairgrounds the targets give points when you shoot em if you shoot the peanut target you will lose -100 points if you shoot the mustachio target it gives you +100 points the other targets give +10 points to unlock the gnome you have to get 750 points

as i said in the other section you have to get 750 points to unlock the gnome, its easier if you shoot the mustachio target keep alert of the peanut target now if you unlocked the gnome some doors where the gnome is those doors will open

you have to carry the entire gnome for the whole campaign and dont worry if you die you will respawn with the gnome, also if you need to defend or kill zombies then you can drop the gnome (recommended to put it into a safe place) then when you are in the concert call the helicopter with the fireworks and get into the helicopter and you will get the achievement

and thats it