It was surprisingly easy.
I'm posting this from Firefox inside Gnome on Athena.
I just had to wait for KDE to finish compiling. I decided to install all of kde (using the kde-meta ebuild that pulls all the components as dependencies) thinking that that was what I had done last time. Well, perhaps it was, but I don't remember it taking 24 hours to compile last time around. Anyway, the reason I had to wait until it was done compiling because I tried starting X using an auto=generated config file and it froze the computer. I tried it with my old one, and it did the same thing. Crashing is not good when you're compiling, because the hard disk is engaged, so I just did other things and installed other software (which is partly why it took that long).
Anway, the easy part was reading the logs from the failed loads, changing the display driver from nvidia (a proprietary driver that I haven't yet installed) to nv (the open source version). I had to make a symlink from /dev/mouse to /dev/input/mice, and I disabled some things that the log had flagged warnings about, logged in as a user, typed 'startx' and, to my amazement, was greeted with the Gnome Desktop environment.
I think just it seemed easy because I knew where to look and what to do this time around.