We seem to have different ideas of what is a "gentleman's knife". To my mind, such knives are small, thin, elegant, and generally traditional. Perfect examples would be AG Russell's Ultimate Pen Knife, and its two bigger lockback brothers. Sticking with AG Russell, the Funny Folder and the Mini Lockback would qualify.
There are many gentlemen's knives, and none of them have thumbholes or plastic. Simple elegance is their hallmark.
I do think that today's gentlemen knives don't need to be made of traditional materials, nor do they need to look traditional. It's still important that they look elegant, and typically not aggressive either.
Nearly all Klotzlis are high-tech designs, and for example a Spyderco Walker isn't a traditional gentlemen knife as well. In the cheaper segment, I think that the Boker Plus Exskelibur 2 is a perfect modern gentlemen knife.