Specificaly GBB? Never knew that sorry.
What about a bullupu pistol kinda thing...or this.
Calico M950 9mm "pistol" with 50 rounds magazine
Or
Calico M100 .22LR (5.6mm rimfire) pistol with 100 rounds magazine. Note ejection port on the right side, characteristic round handguards, front sight and flash hider.
Make it like full auto and it even has a flash hider for your dog to lick.
Coutasy of
World Guns/
Calico @ World Guns