Important Parameters to Check while Chosing a GSM/GPRS Antenna
Here are a couple of things to consider.

An antenna with gain means it has some directionality. For instance, take the example of the satellite dish - it has a gain of many dB but needs to be pointing at the transmit antenna fairly precisely to receive the best signal. A few degrees out can mean barely receiving a signal at all. It works both ways - transmitting and receiving are governed by the directionality of the antenna. More directionality means more gain compared to the theoretical isotropic antenna (that has a gain of 0dBi and transmits equally in all directions.

An antenna with lower gain may perform better in some environments than an antenna with higher gain because the higher directional antenna may be difficult to align.

VSWR ulitmately informs you what the impedance mismatch betweeen antenna and coax might be - a high VSWR usually means either wrong impedance feeder cable or the antenna presents a complex to the feeder and some form of matching circuit is needed.