Mordred, Lancelot Conquesta, and the Guren Flight-Type are the only KMFs so far that can match Gawain's firepower. I think Mordred can exceed it, but again, it's only one Knightmare.
Gawain was able to shut down two armies in R1 because it could fucking CLEAR THE SKIES in a single pass of its Hadron Cannons. It guaranteed air superiority for the Black Knights. So far, Britannia's got only two other Knightmares that can do that, and Lancelot Conquesta doesn't have Gawain's capacity for sustained fire.