How to convert from CDMA to GSM?

GSM and CDMA standards use the same Core Network architecture, only the Radio Access Network (RAN) that is different where different radio access methods are used and different base station that supports each standard is used.

So if a (GSM or CDMA) call is initiated and destined for a (CDMA or GSM) end user , it will be received by its corresponding base station (GSM or CDMA) and transported to the Core Network (which is the same for both standards), then the call will be directed to the other system's base station (CDMA or GSM), which will transmit it to the requested destination.

Hope I answered your question :)

Since you they are two different access methods, you cannot convert normally.
You will need some special tools for the conversion.