Hi Dave. Welcome to the forum!
I think James' analysis is probably spot on. The steels aren't greatly different from each other in terms of their ingredients, except for the 4% Molybdenum that ATS-55 has. That should essentially make the steel harder than the other, or less strong at the same hardness, tho I've not heard too much about brittleness being attached to Molybdenum. GIN-1 has a bit more chromium, which should give it a slight edge in corrosion resistance.
If you're interested, Spyderco has one of the more complete steel makeup charts around. Here is the URL:
http://www.spyderco.com/education/steel ... 3JF2NSDHR4
I'm a little fuzzy on the effects of the elements to the left of chromium in the charts. The metals to the right of chromium form carbides by uniting with the carbon in the steel, during heat treating -- and probably forging. Those elements carbides are quite hard. Vanadium carbides are the hardest, and smallest. Tungsten is right behind. Interestingly, the harder the carbide, the sooner it forms in the sequence of events. Chromium also forms carbides, which are also pretty hard. The property of corrosion resistance comes from "free" chromium, that isn't tied up in the formation of chromium carbides. In general, the smaller the grain of steel, the stronger it is, and the better edge one can get. The harder the carbides, the longer the edge should last. In most instances, the longer an edge lasts, the harder the steel is to sharpen -- in stainless steels. Forged carbon steels on the other hand can have terrific edge-holding capability, and yet be very easy to sharpen, even at a fairly high hardness rating.
At Blade Forums, there is a great deal of information available about steels. There are FAQs on the subject, and many, many threads over the years. Much of the discussion is in the general forum, Blade Discussion forum. There are 4 archives of that, in addition to the current material available in the forum. Use the search function, and you can find a lot to read, and learn. Oh-- there is also another Spyderco forum there, and Sal moderates it too. It's under the manufacturer group of forums.