To compile the sourcecode you need to set the GP2XDEV environment variable and probably edit the Makefile present in the src folder.
This is an example environment:

# export GP2XDEV="/usr/local/gp2xdev"
# export PATH="$PATH:${GP2XDEV}/bin"

=== Installation ===
Put the file "autorun.gpu" and the folder "gmenu2x" on the root of your sd, create the links as described on the documentation present on the website (http://gmenu2x.sourceforge.net) and boot your gp2x.