Your refrigerator has a basic defrost system. Defrost timer, defrost thermostat, defrost heater and harness.
You need a multimeter to test this components.
Slowly turn the defrost timer shaft CW until it stop the compressor, make sure the defrost thermostat is closed, then the heater should come on.
Is the evaporator fan works?
Can you post the picture of the frozen evaporator?
Here are the breakdown diagrams and Freezer Section parts for GE GTS18WCPFRWW - AppliancePartsPros.com