The Buildergat is a Wonder Weapon that can be found in special smashables. If you look on any world you will see a new smashable everyday. You won't see it normally but the smashable is only in secret areas.
The Buildergat gives Players 10 more Imagination and 5 Armor. It can take out 10 Stromlings in one shot, but the gun only has 20 ammo. Waiting with it in your backpack rather than carrying it reloads it quicker.
'Wonder Weapon' and Buildergat are references to the Black Ops II Zombies map 'Mob of the Dead'.