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:
source /usr/local/GNUstep/System/Library/Makefiles/GNUstep.csh

then start your applications with "openapp <name>".