I'm setting up GAP on a Linux PC. The text version goes OK, but xgap doesn't
work: gap starts and no input can be fed to the gap subprocess.
Compilation had worked fine, though. just a minor change in undefined
signals.
To make it worse, I tried to compare that with xgap on a SUN and couldn't
compile it. It ends with

cc -L/usr/openwin/lib -o xgap xcmds.o utils.o gapgraph.o gaptext.o pty.o popdial
.o main.o -lXmu -lXaw -lXt -lXext -lX11 -lm
ld: Undefined symbol
_get_wmShellWidgetClass_get_wmShellWidgetClass
_get_applicationShellWidgetClass
*** Error code 2

If someone has a clue for any one of these problems (mostly for Linux),
please help.

I too have had problems with xgap under OpenWindows. It seemed to spend all it's
time flickering somehow, and never got any work done. Under X11R6 it works fine.

To get it to compile try re-searching libXmu at the end of your
list.