[@Raineh Daze] 
1) by no means is Excalibur on par with a nuclear blast let alone "much worse" 
2) Gilgamesh he doesn't have weapons of Excalibur's level. He has the prototypes of the best weapons. 
3) that isn't the power of Excalibur that was the power of her Magic Circuits flowed into the weapon which amplifies the users magical abilities.