Unknown short-circuit between GND and ANTENNA pins of GSM module
If you measured the resistance between the feed conductors of a dipole antenna you'd see an open circuit - this doesn't mean that something has gone open circuit.

If you measured the resistance of a loop antenna it would read maybe less than one ohm and this doesn't mean that there is a short circuit.

If you measured the input impedance of a dipole at it's resonant frequency it would be 73 ohms but there is no resistance in the wires to speak of.

Antennas cannot be judged to be working (or not working) with a simple DC test unless the type of antenna is known. I'm not about to read thru all 60 odd pages of the document linked in your question so if you want to know the real answer please find the relevent sections and maybe post the details.

Because this is also a radio module i.e. will have an RF output fed via LC filters to possibly a patch antenna, the inductors that shape the frequency response may be grounded and thus give the impression that the pins are faultily connected to ground.