If you have the squashfs mounted correctly, when you type "df" in a terminal, you should see a line like:
/dev/loop0 numbershere samenumbershere 0 100% /opt/native/arm/3.4.6-xscale-softvfp
and when you type "gcc" in the terminal, it chould say "gcc: no inout files."
To mount the squashfs, just do a:
mkdir -p /opt/native/arm/3.4.6-xscale-softvfp
mount -o loop -t squashfs /path/to/zgcc-3.4.6.img /opt/native/arm/3.4.6-xscale-softvfp
You can get zgcc from http://distro.ibiblio.org/pub/linux/distributions/pdaxrom/
To compile a Gnome app (without a Makefile or anything), you probably need to do something like this (assuming it's a C file):
gcc -o myprogram myprogram.c -lgnome -lgtk
The linker flags may be wrong there, but that's the general idea.