The DTM allows a MS to do a CS+PS transfer, in parallel.
The Gs interface allows to page a MS which is in PS transfer.
Those 2 features are totally independant. However, both are needed to allow this scenario :
the MS is doing a PS transfer
the MSC needs to page the MS because of an incoming call
the CS Paging will go through the Gs interface
the MS will then establish a DTM connection (CS+PS), allowing to keep the PS transfer and answer the call at the same time.
Without Gs interface, the above-described scenario cannot take place :the CS PAging will be unanswered.