To get te gnome out you have to score high on the little shooter game. Then the party has to carry it into aevery safe room and to the chopper. You should get the badge in the finale even if u die, as long as someone gets it to the chopper. I recommend dropping it by the stage in the center so you can and run to either side the chopper lands on

Note, it acts as a sole melee weapon. You can keep a gun on you, but if u equip the gun u drop the gnome, so the guy who carries it is pretty much a melee monkey