I think that this is a considerable exaggeration. It was always quite possible to be a gentleman, even an aristocrat, and to be in certain sorts of professions (especially, but not only, a military officer) and even certain sorts of business (in a CEO or director type role), if it did not occupy too much of one’s time, and was not too obviously money grubbing.
Not that any of this is relevant to Bilbo, of course.
Are you talking about hobbit gentlemen or human gentlemen? Because I believe Arnold Winkelried is correct about human gentlemen. Or maybe you mean 2012 not while Tolkien was alive?
I am most certainly talking about human gentlemen, and when Tolkien was alive and earlier, back when the term “gentleman” really meant something (by Tolkien’s own time it had ceased to mean much at all, but his writing was nostalgic for earlier eras). In the 18th 19th and early twentieth centuries, the entire officer class of the British military were gentlemen, if not aristocrats. The more successful doctors and lawyers would also, for the most part, have been considered gentlemen, as well as at least some business men. They might have had some inherited money or land, as well as an inherited status, but some of them still chose to work (or even needed to, to maintain their preferred lifestyle).
Many gentlemen may not have done any paid work, and lived entirely off inherited money and rents, but it is just not true that not working for a living was ever a necessary qualification for being considered a gentleman.
Am I the only one who found Bored of the Rings puerile and more or less the opposite of clever and entertaining? I mean…puns? Really? That’s all you’ve got? Let me go fetch a copy of a Xanth book or something.