I think it would be good idea to divide this question on two subquestions:
1) How to use private API
You can check following questions for info on this subject:
How to use iPhone SDK Private APIs
http://aralbalkan.com/2106
iOS Private API Documentation
Getting signatures of private API methods for iOS
If you will have any additional questions on. You can post it here or send me an email (I have it in my profile).
2) How to intercept gsm data using private api?
I don't know.
Three question for youL
Did you really mean "gsm" data (gsm protocol level information) or did you mean wireless data (let say HTTP requests which are send from device)?
Do you need to intercept and modify or do you need just intercept and view them?
If you meant wireless data and just viewing it then try to search for traffic sniffers. Some of them should work on iOS.
One more note. As soon as I hear "intercept" my guess would be that you will be better off with jailbreak tweak (vs private API). Private API's are limited in usage. Is this ok, if your app will work only on jailbroken devices?