It is indeed a "stereo" 3.5mm jack. However, actually getting audio in and out of the GSM Shield V2, for example to make/receive calls, is an entirely different headache I've yet to overcome.
Interestingly (though it hasn't helped so far) v1.7.8 of the IDE has better GSM examples (for instance the receive SMS manages to receive more than one - the old one receives one then locks up) and a new voice example "ReceiveVoiceCallGSM2".
Well, apart from v1.7.8 having utterly vanished (it's not in the list of older IDEs to download) and the updated examples in the current v1.8.5 are old ones, it's back to square one with that.
The v1.8.5 IDE I installed at the weekend was for Linux (previously I'd always used Windows) so I've yet to see if this suffers from the same flood of compilation/build errors the Windows v1.8.5 IDE suffered from when trying to build for the GSM v2 shield.