Unless things have changed since I was last in GC for pre-flight dinner, most of the food options are buffet / ready for you to help yourself with one or two 'stations' where you can order e.g. pasta made to your liking. I've never been a fan of the latter so most of the food is there waiting for you on the buffet so I would not budget any waiting time for food prep.

Therefore it's a case of how long you want to take to eat and how much food as others have said. Personally, I don't like rushing so avoid cutting it fine so I would aim to get to the airport 120 mins before departure so that I am in the lounge 90 mins before dept giving me an hour to eat, relax, etc before boarding at T-30 ish. That includes contingency time.