Or he would blast you, but you could dodge out of the way - like Adam Warlock did in 'Infinity Gauntlet'. Galactus is not above swatting at gnats if they annoy him, but he generally gives up pretty quickly - he has other fish to fry. Doom, on the other hand, will not rest until he has tracked down the gnat and SQUASHED it! He probably spends entire days stalking, say, an annoying mosquito through Castle Doom until it is in the exact right position to be stomped upon by his metal boot.