Difference between Millie and Para2 is mainly size of course, but also liner lock versus compression lock, right handed tip down clip only vs. four way clip, smaller lanyard hole on Millie, More pronounced 50/50 choil on the Para2 and of course the higher price on the Military. Both are made in the USA, which is important to some people.
The Gayle Bradley uses more expensive materials than the others, with the Carbon fiber and M4 steel, which I assume is the main reason why it is more expensive. It would probably have been even more expensive though, if it had been made in the USA and not Taiwan. This is of course no big deal at all practically, since excellent knives are made in Taiwan for Spyderco. The Gayle Bradley also has a high hollow grind compared to the FFG of the Millies.
The Gayle Bradley also has a slightly oddly shaped handle, which was the reason I passed on it (and later decided to point my phasers on the Para2). IMO the finger grooves on the handle of the GB look very shallow, increasing the risk for slipping onto the edge with your fingers.
This is just from my observations of pictures of it though. I haven't actually held the knife in my hand, so I don't know how it actually feels in reality. What I do know for sure is that my Para-Military of the second generation is the best folding knife I've ever held in my hand.