Hi Jake,
you first need to install the dependencies and build tools.
If you have that, compiling/installing the GCS is no different from how it's done on Debian/Ubuntu, as that is always built and installed from source.
Apart from common tools like git, make, gcc, g++ and development libs like libusb, libpcre, libsdl you need gtk2 with glade and then OCaml:
For OCaml you could try using what is there in Arch already or immediately try installing ocaml itself and some libraries needed (ocamlfind, xml-light, pcre-ocaml, lablgtk, ocamlnet) via OPAM.
This toolchain replaces the paparazzi-arm-multilib debian package since v5.0 anyway...
Hope that helps for now....
It would be great if you could document the packages and steps you needed.
Cheers, Felix