While it lacks a certain ambiance, dining in the Gourmet area is certainly one of Paris' great bargains. For less than 10 euro you can get a gourmet sandwich, of say, manchego cheese and freshly sliced Iberian ham, piled on bread baked on site by Maison Kayser, one of the best bakers in Paris. You'll find meals available at comparable prices on counters throughout the market -- and there's also seating available in the seafood area.
There are several restaurants outside the Gourmet area proper, notably, Bar Rouge, which is right in the wine library, and the casual Lafayette Organic, a healthy food restaurant that opened in December 2010. See the full range of restaurants available in Galeries Lafayette.