Garage door controllers uses RF for sure, and only the very old models used a fixed frequency to open and/or close the doors. The usual controller uses a variable frequency that must "negotiated" between the remote and the receiver in order to avoid hijacking your code.

Anyway, N900 has various RF transmitters like Wi-Fi, Bluetooth and even FM Transmitter, but they must use a defined frequency range, thus I guess it is impossible to make it open a garage door since the hardware can not transmit in the needed frequency. I think garage doors remote controllers here at Brazil operate at aprox. 400MHz

Didn't know GSM garage controllers tough...