Interfacing a GPRS chip (WISMO218) with an RF antenna. Calculating line width for the right impedance
It's called a Coplanar Waveguide With Ground. To find impedance, here is a calculator.

This is a cross-section through the line. The red is the RF track, the blue is ground. The dotted blue lines are vias, you should put them every 5 to 10 mm along the sides of the ground.


If I assume you have a two-layer board,
Er=    4.2 (good guess for FR4 if you don't know better)
Board= 1.5 mm
Track= 2.1 mm
Gap  = 0.5 mm  

You will have a 50 ohm line.  There will be other solutions with wider tracks.

Deviations of 10% from 50 Ohms will not matter in this application. But as they say in the datasheet, keep the track short! If I recall correctly, at 900 MHz, the attenuation is 3 dB in just 5 or 10 cm. 1800 MHz is be worse.

Don't forget to put plenty of vias down the sides of the CPW, to the ground plane underneath.