GURPS and d20 are based on such different assumptions that your best bet is to convert by comparing Gyrocs to other weapons. Look at GURPS stats for shotguns, heavy assault rifles, and grenade launchers. Then look at the T20 stats for the same weapons and take your best guess.
Amusingly, Gyrocs are broken even in GURPS rules--you cannot reproduce them using the G:Vehicles metarules for firearms.