hey Hi….
instead of AT89S52 use 2 PIC16F887/886 controllers.
if you are using ZigBee modules of Microchip then you can have readymade stack from its web site…..now you just need to modify Application layer according to your application….
so use 1 controller for interfacing ZigBee module & another to interface GSM modem & then write a code to interface both controllers…..
you can do in such a way like if you some input from ZigBee then the output of its PIC controller should be INPUT of GSM systems input….
for interfacing circuit study the educational interfacing kit of PIC controller given by Microchip & modify according to your need…
hope this will help you…..