It looks like you haven't sourced GNUstep.sh. What you need to do is figure out where it was installed and then source it before running make. If you used make-2.0 with the standard gnustep filesystem layout it will be under /usr/GNUstep/System/Library/Makefiles/GNUstep.sh (I think).
I generally source this file under my .bashrc file, this way I never have to worry about it.