i believe gilgamesh was a lot more difficult than Yiazmat at the time i fought him. i managed to kill Yiazmat only in my 3rd visit to him. Vaan with berserk, haste, protect, bravery, bubble belt and Ragnarok. Fran haste protect, shell, bubble, cat-ear hood and Artemis bow with assasin's arrows. And Penelo haste, protect, shell, bubble, sage's ring and cloud staff. i had Vaan attack him physically, penelo casting Scathe and gambit hp<70% curaja and Fran attacking with her bow, casting bubble and hastega when needed and gambits ally=stop chronos tear, ally=KO phoenix down and ally=petrify gold needle. he took me about 7 hours in total to take all his HP away. the only annoying part with Yiazmat was his high HP. gilgamesh in the other hand (i was too low level for him) i tried around 20 times to kill him. and the worst was that i didn't know about the genji staff he had and i didn't steal from him.
Gilgamesh>Yiazmat