Oh and for the record I would like to point out that bb projectile dispensing devices are neither guns nor firearms. A gun would refer to an artillery piece sized weapon where as a firearm would propel the projectile by deflagration (that is the confined burning of a propellant). That is unless you modify your GBB to ignite the propane as it come out, Bad idea.
P.S. care to tell me why your link is blocked by my web filter telling me it contains items of a sexual nature