I've replaced a wheel-dead "Microsoft InteliMouse PS/2 Compatible" with
a new "Microsoft Mouse Optical USB and PS/2 Compatible." Didn't want to
fiddle with USB and plugged it into the PS/2 socket with the adapter
included with the thing. I have gpm starting with

/usr/sbin/gpm -m /dev/mouse -t imps2 -3

and, on reboot, got a message from gpm about incompatible protocol. If I
restart gpm with "rc.gpm restart" it starts all right (with no error
message displayed) and it worked fine with KDE with or without the
message from gpm.

So, is this thing a "imps2" or should I be using some other protocol? I
looked but couldn't find anything helpful.

I should have been suspicious when I opened the box and there was an
installation disk included with the thing, but there you are.