I have no idea if this might be helpful to anyone, but I was having some problems getting the GPM mouse support to work on this computer. I tried a regular PS/2 port, and after a couple days of reading manuals and trying various protocols, I finally found some that worked. I then realized that X11's interface with the mouse, through GPM or not, didn't work right when GPM was running. I have to stop GPM to use X11 properly. I guess my motherboard's PS/2 port doesn't work right. I then started using a USB mouse and X11 with the IntelliMouse protocol on /dev/gpmdata, and it worked a lot better but still had problems. Until a few days ago, it was still really jittery when there was a high load or I move it too fast. I tried sharing /dev/input/mice between GPM and X11 at the same time, and it worked perfectly.
So, if anyone is having that kind of problem, they might be able to try using a USB mouse and having GPM and X11 share /dev/input/mice at the same time. Unlike with /dev/psaux, sharing /dev/input/mice between GPM and X11 seems to work well.