Your fridge does not have a defrost timer is uses what is called an adaptive defrost control.
It is located in the control panel of the fridge it is a little electronic control.
If the fridge is not defrosting you will have to see if you can manually initiate a defrost.
To do this open the fridge door and push the light switch five time with in six seconds.
If the heat and defrost thermostat are in working order the fridge will defrost.
Here is a link to your fridge model glrs237zdw2. | 
Locating the control defrost timer 
