All you have to do is pick the gnome up, I would actually suggest a peashooter and sprinting towads the first one that spawns and hope you get there before anyone else, or you could just follow a team mate around and wait for them to die. Also you can just kill the enemy and pick it up.
I was actually trying to do it and got on here because it didn't pop up and went by the other guide and tried it again. It still didn't pop.
So, make sure you have the ZOMBOS PACK DOWNLOADED cause even if you don't. You still get the maps and game modes, if you don't download it, you don't get the achievements. Don't be like me and play a bunch of games and wonder why your achievement didn't pop up.
Once I downloaded the dlc, all achievements I earned without the dlc downloaded unlocked for me. Just play a match