Gniezno's eating scene is modest but genuine. The restaurants around Stary Rynek serve standard Polish fare — żurek, pierogi, kotlet schabowy — at prices that feel reasonable after any time spent in Warsaw or Kraków. Nothing is particularly ambitious, but nothing is bad either. This is a city where people come to see a cathedral and eat lunch, and the restaurants have calibrated themselves accordingly.
A few cafes around the square are good for coffee and cake before or after the cathedral. If you're arriving by train and have a half-day, there's plenty to keep you fed without any prior research or reservations.