I'm not using gpm.
My XF86Config-4 has this
Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/psaux"
Option "Protocol" "ImPS/2"
# Option "Emulate3Buttons" "true"
Option "ZAxisMapping" "4 5"
EndSection
before I installed it I found a page at http://www.koala.ilog.fr/
describing how to use a wheel mouse with X
Bob