GSM is using multiplexing which uses different channels on available bandwidth for connectivity. In most cases multiplexing uses either Frequency Division Multiplexing or Time Division Multiplexing. At the other end CMDA uses multiple access scheme that means it can allot to multiple users with given bandwidth. CDMA technology is more safer than GSM as in CDMA it provides unique codes for each conversations between two users.
I will explain you with more easy example to understand these two different technology by following examples.
If there are 20 people in the one room who are looking to talk with another person but without getting disturb, so there are three ways to do this.
1. Only two people can stay in that room and other 18 people leave the room for some time, or you can say for five minutes. For that five minutes they both can talk with each-other and after ending their conversation they can go out and next two people can come in the room. This process done by all 20 people and again the first two people come to room in the cycle for continue talking. This called TDMA (Time Division Multiple Acess).
2. Second thing they can do, divide that room in same part for twenty people, like two people in one room and ask them to talk with their partner in the respective area. This called FDA (Frequency Division Multiple Access).
3. And the third thing is, they can stand anywhere in the room and talk with their partner. They all not need to wait for their turn or their time slot. But they should speak in different languages. This called CDMA.
I hope that this article helps you to understand both technology.