Here are some advantages of GSM :

GSM is mature, this maturity means a more stable network with robust features.
Less signal deterioration inside buildings.
Ability to use repeaters.
Talk time is generally higher in GSM phones due to pulse nature of transmission.
The availability of Subscriber Identity Modules allows users to switch networks and handset at will.
GSM covers virtually all parts of world so international roaming is not a problem.   _{Your Comment}