When omni directional behavior is not required, first start with dipole/patch over a ground plane to get some gain. When using a real microwave diode (BAT15, or equivalent), you can get significant increase of sensitivity by matching the diode to your source. In a parallel equivalent circuit, the parallel resistor will be in the kOhm range.
An end-fed half wave dipole has high impedance already. When placed above (parallel to) a ground plane it will increase to kOhms also, so you only need to compensate the diode capacitance with an inductor. The transmitting band for GSM is not very wide, so you can accept high Q-factor.
I would recommend the dual diode voltage doubling approach, as you can use a diode at the rectified output to prevent reverse breakdown. Reverse voltage rating of microwave diodes is about 4V. to get full DC EMF of the diodes, you need a voltmeter with > 1MOhm input resistance.
10mVp RF across the junctions of a voltage doubler, results in about 2mV DC out. 10mV into about 2 kOhm equals about -46 dBm. Every time you halve the RF input voltage, the DC output drops with factor 4. Also note that GSM uses 1/8 duty cycle, so the actual output voltage will be less.