<<I want to write a command line app in Objective C that needs to run on Linux and Mac. I read I can use Foundation on Linux using GNUStep. Do I need the whole of GNUStep, or can I just depend on Foundation?>>
In my experience, you need both <Foundation/Foundation.h> and <AppKit/AppKit.h>. You also need all the components that are listed on the website: make, base, gui, back. Ubuntu plays nicely with GNUstep and loads all the pieces for you. My attempts at downloading the individual pieces with other distributions were frustrating and I never did get them put together correctly.
<<Why do I need to source GNUStep-make?>>
GNUmakefile will make your life a lot easier, in my opinion. I could not get the gcc command line to work correctly (in addition, it's a long command). The GNUmakefile that I use looks like the following and is placed in the same folder as your source code. Just type "make" on the command line after migrating to the folder where your code is located and it will help you do the compile.