Is it possible to use SIM900 GPIO to control relays without using external MCU?
The GPIOs in sim900 module is meant to be controlled by AT commands through serial port ofcourse. 

if u see the AT command manual of sim900 you will find the following at command to control the GPIOs 

AT+SGPIO=&lt;operation&gt;,&lt;GPIO&gt;,&lt;Function&gt;,&lt;level&gt;


following is the Description to the above function!



P.S. i know this question is about 20days old and he might have already got the answer.I'm only sharing because it may help other.!