One thing that stands out immediately is that it is strictly medieval, with all "New World" foods such as potatoes, corn, tomatoes, pumpkins, chocolate, vanilla, and tobacco conspicuous by their total absence (at least in all of the references I checked). This is a bit surprising, both because Gary generally wasn't hung up on "anachronism" (and in his later years advocated moving the default technological base of D&D forward to approx. 1650 for everything except gunpowder) and furthermore because he posited a class of cosmopolitan inter-planar travelers with knowledge of other worlds, including modern-day earth, and surely could have brought back such items (similarly to how Gord purchases several bottles of 1947 Chateau Margaux Margaux from a wine merchant in Weird Way), but the consistent absence of such items can only have been deliberate. With that in mind, and aided by several Google searches, I've filled in a few blank spots in the culinary landscape based on typical European medieval cuisine.
Does any of this matter or make a difference when playing D&D? Not really - as long as you know that a "merchant's meal" costs 1 s.p. and a week's supply of rations costs 3 g.p. for "standard" or 5 g.p. for "iron" per the Players Handbook it probably doesn't affect the game to know what exactly they consist of. And yet, added detail can also make the game more immersive, and help the players to picture the imaginary world. Going into exhaustive detail on every meal the characters consume is undoubtedly overkill, and yet the bill of fare at the Inn of the Welcome Wench with its list of exotic wines and brandies is still fondly remembered almost 40 years later as the kind of detail and flavor that made Gary Gygax's version of D&D so evocative.
Breakfast: bread (loaves, rolls, muffins), gruel/porridge (semolina, groat clusters), oat cakes, herbs, berries (whortleberries ("European blueberries"), lingonberries, blackberries, black currants), jellies, honey, cream & butter; herbal tea* or small beer (There's no mention of bacon, eggs, breakfast sausages, or ham - presumably in the Flanaess such hearty breakfast fare is consumed solely by hard-laboring farmers and not by city-dwellers or travelers)
Poor fare: gruel, soups, stews ("slumgullion"), hard black bread; small beer or sour wine
Dinner/supper - common inn and tavern fare: loaves of bread, puddings, soups, stews (ragout), steak and kidney pies (hot at dinner, cold at supper), smoked meat and fish, roasted meat (pork, mutton), roasted fowl (capon), sausages, fresh fruit and nuts**, boiled eggs, cheeses, butter, honey; beer (small beer, ale, stout, milk stout), herbal tea, honey mead, wine, mulled wine
Dinner/supper - rich or elaborate fare: fresh fish (poached salmon, stuffed trout), exotic seafood (smoked eel, boiled crayfish in drawn butter, crayfish soup), roasted venison, roasted or stuffed fowl (squab, pheasant, goose), fresh greens and vegetables (mushrooms and truffles, radishes, pickles, scallions, salads), spices (pepper, saffron, ginger), rare and imported cheeses***, butter and cream, fresh fruits and berries, tarts (berry, nut, mincemeat), iced cakes; rare and imported wines and brandies****, whiskey
Travelers' fare (i.e. "standard rations"): hard sausages, dried fruit, dried fish, wheat loaves, cheese, pickled vegetables and eggs (iron rations = jerky, hard tack, hard cheese, dried nuts)
Regional variances: In Gary's works the menus are mostly the same whether the meals are being served in Stoink, Urnst, Hommlet, Veluna, or Greyhawk City. Some of that is presumably due to the characters typically dining in inns and taverns, which are likely to be more similar to each other than if they were dining in local homes (noble or peasant). Also, those locations are all centrally located along the tributaries of the Nyr Dyv, and had Gary gone into more detail on the cuisine of far-flung locales we might have seen more variety. To step outside of this "canonical" baseline, the notion of Cultural Approximations in Greyhawk suggests some fairly obvious regional specialties - waffles from the Duchy of Urnst, raclette from Perrenland, breaded veal cutlets from Veluna, haggis from Geoff and Sterich, etc.
*Tea is mildly anachronistic in comparison to the other mentioned foods (since it wasn't commonly introduced to Europe until the 17th century) but nevertheless Gary mentions it frequently, and even includes a couple of dedicated tea-houses. Characters consume a variety of different herbal teas including alder-root tea, bark tea, blackberry tea, lingonberry tea, and an unspecified "smokey-flavored tea," but never common black or green tea
**based on the list of common trees in the World of Greyhawk Guide pp. 6-7: apple, apricot, cherry, chestnut, fig, galda (Oerth-native), grapefruit, kara (Oerth-native), lemon, lime, mulberry, olive, orange, peach, pear, pine, plum, usk (Oerth-native), walnut, yarpick (Oerth-native)
***Gary describes and named several such cheeses, including smoked Okelard cheese (presumably equivalent to gouda), Kettite goat cheese, Perrenlander cheese (equivalent to Swiss), Wickler from the Yeomanry (a blue cheese), and Djekul - a creamy, smelly cheese from the land of Fruztii (presumably equivalent to something like Pont l'Eveque). Surely there are many more such cheeses in the Flanaess, making this a ripe (ha!) area for further individual development
****A wide variety of wines are named and described, giving us a pretty solid sense of the wine economy of the Flanaess. The Rhennee typically drink a harsh red wine but favor fine wine from Caporna (wherever that may be [EDIT: a town in County Urnst, on the Artonsamay River]). Likewise, the Paynim tribes drink pungent date wine, but value the wine of the Chepnoi people of the Sulhaut mountains. A strange, mildly addictive black wine comes from the Pomarj, but production of it has declined since that land was conquered by humanoids. The major wine-producing areas are Urnst (white wine and special aged brandy) and Keoland (golden wine, amber wine (served chilled), and brandy). Furyondy and Veluna produce comparatively fewer wines, but theirs are among the most celebrated - Furyondian dry white and emerald pale, and Velunan fireamber. However, the rarest and most celebrated wines of the Flanaess are produced by elves - Sunndish elves produce lilac wine, the elves of Celene a ruby wine, emerald wine (served chilled), and nectawine (made from moonberries harvested only when both moons are blue), while the elves of Ulek produce both a heady, sparkling violet wine and their unique "elixir" liqueur. Even the drow produce wine - a black wine with an earthy smell and taste like nothing else that is so strong that consuming a single gill (4 oz.) will make a human tipsy.