The Victoria Cross is awarded for Gallantry above and beyond. Garland and Gray were in a position to 'avoid' the heavy flak but chose not to...the gunner was indeed considered "part of the Aircraft" and had no say in the prosecution of the attack - he died carrying out his expected duty. Yes he was brave - heroic even, but they all were: it was the decision process that went above and beyond and not just being there.
Strange, Ken Cambell's navigator, James Scott, didn't get a VC as well when they torpedoed the Gneisenau. Considering they found Scott in the pilot's seat when they recovered the wreck, he could be said to be particularly hard done by. I imagine he too chose not to avoid the heavy flak just as much as Gray did not.