I couldn't grok Alternity Gamma World. The setting was great, but I didn't get the rules. I've read that it has many design elements that were precursor to 3e, though.
It does, a bit. You can see the beginnings of 3e's skill system in Alternity. The entire Alternity system is based off of the skill set, which makes the game feel more homogeneous than 3e does, with its numerous class abilities, BAB, and magic systems. One thing it does differently is that it does not pretend that all skills are made equally, with some skills requiring more points to improve than others. There are also class skills, but the penalty is not so severe to dip into skills that belong to other classes, and trained only skills are present also.
The die mechanic is much different though. While you still make frequent use of the D20, you want low rolls, not high. The GM assigns bonuses and penalties to the roll, (In the form of polyhedral dice to add or subtract from the roll) but DC's are inherent; a player will know right away whether his roll was a success or not, because it's right there on the character sheet. There are also degrees of success and failure as well.
The game does not assume you will be using miniatures to play. With no concerns over how the characters might move on a grid, or how much space a character takes up on the board, AOO's and the like, it is more assumed that you will not be using miniatures. One thing that might be hard to grok nowadays is that the game seems to assume you know what constitutes an "action". In most cases this would constitute a skill check, but is standing up an action? Opening a door? The game does allow you to move and act, but there is no list of free actions, move equivalent actions, and the like. (that I have been able to find, anyway)
I almost wonder what a 2nd edition Alternity might look like, with more modern game design elements in it.