I can't figure out for the life of me what the chef lady in Daea wants. She says she wants an exciting ingredient, and I've tried giving her everything. Those spiny fruits, the oranges (those are rare), those ancient canned goods, those nut things from the forgotten forest, even that blue slime jelly stuff that's crazy rare. What does she want?! I can't think of anything I've seen in the game that's more rare and exciting than some of those latter things.
Rukariouzu
Okay I've found out why I couldn't find it. After posting my last comment I decided to come off for a while and watch Netflix. After the episode I was watching had finished I thought I may as well give it another go.
When I loaded the game back up, I went to the Great Walls and there Gill was. As far as I can tell, you have to possess 41 moonstones, then reload the game for Gill to appear.
I hope this helps people who were going through the confusion I was, and thank you for your help @nggroundsplayer347