This is hardware specific... what does your GSM modem support? As Kim stated, AT commands are a common way to communicate via serial port to a GSM modem, but its really up to manufacturers what kind of interface they will provide.
So find a modem that you like, look at its specifications, and design your system around that.
Here's a GSM AT command guide:
http://www.arcelect.com/GSM%20Developer%20Guide%20-%20GSM%20AT%20Commands%20-%20Rev%20%20A.pdf[
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