Gilgamesh wasn't too hard IMHO. But I didn't use the book until later in the game when I got stuck and did the main storyline first before doing all the sidequests and hunts thus I was at a higher level already which probably helped out. I know when I fought Vayne my main character was already Level 68. So I must have been in the 70's already when I fought Gilgamesh.

I love in the end when he gets beat he sneaks up and tries to get his sword back, then runs off. What a tool! LOL.