FSviewer is not a GNUstep application. It has a similar look but does
not use any of the GNUstep supporting structure and is no longer being
actively developed. That explains why you could not find openapp. If you
want to use FSviewer then you can probably just drop a symlink into
/usr/bin and it will start like any other apication. If are interested
in GNUstep and want to see the real-deal take a look at GWorkspace:
http://www.gnustep.it/enrico/gworkspace/
GWorkspace is being actively developed and I use it every day. You will
need to install the GNUstep base/back/gui to make it work but I believe
that is all well supported under BSD.