I think everyone has their own definition of what makes a "gentleman's" rifle.
First, who is a gentleman?
1. A civilized, educated, sensitive, or well-mannered man?
2. A man with an independent income who does not work for a living?
3. A man of noble birth or belonging to a family of high social station?
4. A man whose conduct conforms to a high standard of propriety or correct behavior?
IMO the kit a person carries does not necessarily define them. You see miscreants in court, dressed up in a suit and tie - their clothing certainly does not make them a gentleman.
I would consider my best friend a gentleman. He hunts with a McMillan stocked, trued SS Remington 700 with a Krieger barrel - all put together by our very own Guntech. His firearm is a tool and he has it fashioned in the way it will serve his needs best.
In some ways the question reminds me of a "gentleman's knife", which is something you might carry to a fancy dinner, in the pocket of your suit. It would be quite slim, classy and elegant. So is a "gentlemen's rifle" just something classy, elegant and timeless? If that is the case I would consider, new, a Merkel single shot rifle. In my eyes a person carrying a single shot rifle has an air of confidence in themselves and their kit. For used, any pre-war Mannlicher Schoenauer or Oberndorf Mauser.
If you end up with a Mannlicher Schoenauer then you should also take a copy of Hemingway's
Islands in the Stream.
And Old Man and the Sea.
“A gun is to shoot anyway, he thought, not to be preserved in a case, and this was a really good rifle, easy to shoot, easy to teach anyone to shoot with, and handy on the boat. He had always had more confidence shooting it, as to being able to place his shots at close and moderate range, than any other rifle he had ever owned and it made him happy to pull it out of the case now and pull back the bolt and shove a shell into the breech.”
—Ernest Hemingway, Islands in the Stream