To use GNUstep you need to source the envoroment in your shell:
for sh/bash:
. /usr/local/GNUstep/System/Library/Makefiles/GNUstep.sh
for csh:
. /usr/local/GNUstep/System/Library/Makefiles/GNUstep.csh
then start your applications with "openapp <name>".