To my mind a "gentleman's" blade betokens a higher than average level of work and materials; gold, silver, precious stones, ivory, etc. There are a lot of good, small, pocket blades out there, but I would not list them as "gentlemen's" blades. For example: here's a perfectly good small blade made by Pease and carried quite a bit buy me.
This little knife was specifically made for pocket carry by Sheehan (but I never really liked the sheath so I don't use it):
This little Dunn was "borrowed" by a cousin who fell in love with it and uses it on his trap lines all the time:
Buck made these little Ohta's as a special, but they don't seem to have taken off. (I have several by the man and his workmanship is superior. One of his top end "bamboo" series could definitely be a gentlemans knife.):
This pretty little thing is more of what I envision when I hear the term, and it's one of the more copied patterns around. Still, what sets this apart are the materials and engraving:
Here's my basic test; If you can picture the knife being pulled out of the pocket of a white linen suit on the front porch of the county courthouse by a judge or Kentucky Colonel, and you know it was special to the point of being made to that person's specification, then it qualifies. Family heirlooms met this rule; normal production knives rarely do.
And since he's been mentioned, here's a nice little pocket blade Don Cowles was nice enough to make for me: