GSM can be used with Gomunicator, which is in a very early alpha stage or follow the instructions below.
USE THIS WITH CAUTION!!!
GSM modem works . You have to prefix each command with 0x02 0x16 and postfix it with 0x16 0x02. Use the FFUART (/dev/ttyS0) at 115200 bps.
To register on network, you need:
-at+cfun=1 // gives an error, but works
Dial out with
-atdt123456789; (the semicolon is important)
See HTC_GSM for more details.