For capacity purpose some people make this comparison saying GSM = 25 KHz BW but I agree with Pix its not the right way of doing the comparsion. During a normal burst all 200 KHz spectrum is available to 1 user and on the analog side only 30 KHz is available.
In GSM 20 ms speech frame is interleaved and sent over 8 separate half-bursts. If my memeory is right the 20 ms speech frame after source coding + channel coding and bit puncturing, etc at the end yields 456 bits and these are sent on 8 half bursts (of 57 bits). The 8 TS chosen in GSM is pretty much tied to 20 ms sampling period.