Those look like they are from detonation not dry firing, dry firing usually damages springs not seals....Air Rifle Headquarters sells seals, get a Gamo Tesla Seal, depending on how may rounds through it you probably should get a spring too...
Dry Firing won't really hurt a gas ram, it's not affected by the shock and recoil back through it like a coil spring is, main seals can get a little mushed but not enough to affect the performance....