You have what is normally called a "bicycle pistol". Sometimes called a "velo dog", but velo dog is a brand name. Like saying your vacuum bottle is a Thermos or your paper nose wipes are Kleenex.
Back in the good old days, before Dr. Pasteur invented rabies vaccine, getting bit by a dog was something you did not want to happen. If he was rabid, you were going to die. Unpleasantly. So bicyclists got in the habit of carrying small pistols, while riding out in the country, and when a dog came charging out of a farm yard to bite 'em, they'd shoot it. Eventually guns were designed just for this purpose. Folding trigger and no triggerguard allowed them to fit in a pocket easier.
Bicycles used to be called "velocipedes", which his how "velo dog" got its name. Velocipede dog gun.
They were fairly inexpensive, and were made all over the European continent. I don't know who your JGA is/was. These is a company JGA. J.G. Anschutz. German maker of fine sporting rifles. They used to make revolvers. That might be what it is. If so, I would suspect it would be worth more than the average bicycle gun.
Value on an average one, in good shape, would be less than a hundred dollars.