Designing a GSM PCB antenna
There seems to be a remarkable lack of relevant on-web material. Maybe just hiding.

This looks highly apposite Design of New Multi Standard Patch Antenna 
GSM/PCS/UMTS/HIPERLAN for Mobile Cellular Phones with an interesting appearance



And tri-band !!!





GSM Dipole antenna - very informative

Designing a GSM dipole antenna 



Commercial product.

LOOKS simple.
LOOKS good
LOOKs can be deceiving :-)



GSM Pentaband antenna

They say:


800/900/1800/1900/2100MHz 

Omni Directional 1/2 Wave  

Miniature 42 x 42 x 1mm

VSWR &lt;3.0

RG178 Coax 50Ω Impedance

2-3dBi Gain (nominal)

Vertical Polarization

Admitted Radiation Power 1W




The free marvellous NEC RF software will probably do what you want.

Links and intro to NEC here

Have a look here RF/Microwave Tools and here Tools and calculators - with RF writ large 



If you have enough $ - Agilent ADS


Advanced Design System is the world’s leading electronic design automation software for RF, microwave, and high speed digital applications. In a powerful and easy-to-use interface, ADS pioneers the most innovative and commercially successful technologies, such as X-parameters* and 3D EM simulators, used by leading companies in the wireless communication &amp; networking and aerospace &amp; defense industries. For WiMAX™, LTE, multi-gigabit per second data links, radar, &amp; satellite applications, ADS provides full, standards-based design and verification with Wireless Libraries and circuit-system-EM co-simulation in an integrated platform.