Connect SIM module to a Gumstix?
Depending on the gumstix module that you have, if you are using embedded linux with a board that has USB host, then you may be able to use a USB GSM modem - you stick the SIM card into it and then the module into a USB slot on a gumstix board.  You would have to find drivers or write your own.  A quick search finds that some people have had success using a particular model due to driver compatibility.

There are also a number of GSM modules that you can interface with using a UART channel.  They are typically spoken to using an AT command set over a serial port.  If you're willing to explore not having a perfectly clean modular connection (like with the gumstix expansion boards) then there are a lot of options for you.