Thanks to an email that was sent to the gumstix mailing list it was pretty easy to setup the GSM module.

To turn on the module you just need to toggle GPIO100:

echo "out set" > /proc/gpio/GPIO100
sleep 2
echo "in" > /proc/gpio/GPIO100 

The module will signal its on by turning on the blue LED.

Next set up the serial port (ttyS2) to use the correct baud rate:

stty -F /dev/ttyS2 speed 115200

You can now send and recieve AT commands, there are various ways of doing this, the most basic is to just echo a command e.g:

echo -ne "AT\r\n" >/dev/ttyS2

However by using echo you won't get any on the response messages, there are a few options, I personally use kermit which i compiled off OE and installed via ipkg. Now that the module is on there are lots of AT commands that can be sent, I'm tempted to cheat a bit and use gnokii to manage sending the commands as its a nice stable program to use and I've got lots of experience with it.

To shutdown the module all that is needed is to do:

echo -ne "AT^SMSO\r\n">/dev/ttyS2

By installing the kernel module to access the GPS module you'll also be able to access the GSM module through its USB interface, this could perhaps be used to free up the serial port (by cutting the traces but makes sure that you've got a good usb connection first!).