GSM 900Mhz ANTENNA SPECIFICATIONS NEEDED

1. it should present the desired impedance to the transmitter and receiver at 900MHz
2. it must be capable of handling the power, incomiong and outgoing
3. It must not fall apart.

You question is somewhat like asking for the specification of a vehicle without telling us what vehicle it is. It could be a jet fighter, an ocean liner or a bicycle.

Brian.

GSM device in order to be accepted on the network must be certified. Certification body in north America: PTCRB. They require certain minimal TRP (Total Radiated Power) and TRS (total Received Sensitivity) both these parameters evaluate integrated performance of your Transmitter and Antenna (TRP) and your Receiver and Antenna (TRS)
The exact numbers vary from band to band. You can get table of that from PTCRB. Also Some modules manufactures offer these tables. Then you do delicate interplay figuring out how much power from the module and efficiency of antenna you need to fit the bill. If you are in a position you want to maximize antenna efficiency because it's improved performance is free. If you substitute poor antenna efficiency by increasing output power of your Tx you pay for this with increased power consumption and increased spurious emissions. And then you do acceptance tests at one PCTRB - certified facility. If you lucky you pass from first shot. If not - come over I fix you up : )