True, Simonm, some of those words are less useful than they once were. We don't hang criminals on gallows today as in Fielding's time, so "gibbeted" has lost much of its power. Calibash and calipee come from an era when turtle meat was a staple. Just as beef lovers today favor one or another part of a side of beef, turtle lovers distinguished between the flesh attached to the upper shell (calabash) and the flesh attached to the lower shell (calipee).
With a better knowledge of classical history than we have today, 18th century writers were fond of using classical metaphors. While we might call a heavy eater a glutton or a "pig," Fielding chose "Heliogabalus," a Roman emperor known for gluttony. Using this word had the added advantage of imparting an image of a great obese fellow.
Writers today would probably prefer "charitable" to "eleemosynary," but I would argue that the later choice fitted Fielding's meaning better:
"An author ought to consider himself, not as a gentleman who gives a private or eleemosynary treat, but rather as one who keeps a public ordinary [eating house or tavern serving regular meals at set prices], at which all persons are welcome for their money. In the former case, it is well known that the entertainer provides what fare he pleases, and though this should be very indifferent, and utterly disagreeable to the taste of his company, they must not find any fault: nay, on the contrary, good breeding forces them outwardly to approve and to commend whatever is set before them. Now the contrary of this happens to the master of an ordinary. Men who pay for what they eat will insist on gratifying their palates, however nice [fastidious] and even whimsical these may prove, and if everything is not agreeable to their taste will challenge a right to censure, to abuse, and to damn their dinner without control."
We can see that in the contrast between the two situations Fielding is presenting, "eleemosynary" works better than "charitable." "Charitable" might have given the reader the false impression that the meal is being provided to the needy, whereas Fielding is speaking of a dinner for invited guests.
"Ragoo" is just a different spelling of "ragout," which is still in use today.. "Victualler" is now out of favor, unfortunately so because it provides in one word what we now have to convey with two or more ("restaurant operator"?) or the awkward and to many, pretentious, "restauranteur."
"Viands" [food; fare] is out of favor, with probably not much of a loss, but I think I could make a case for the continued usefulness of the other words on your list.