Can you remove the rear panel you see the ice buildup on the evaporator? I believe your refrigerator has a defective defrost element. You need to unplug refrigerator remove the panel and see what type of frost pattern you have. Here is a link to what you may see behind that panel=>
HERE This is a link to what you said you see with frost on the rear panel=>
HERE Can you confirm that your refrigerator model number is GSL25JFPBS? Look around the fresh food section door for your model number. Here is a photo
HERE with the locations number 230 is the defrost heater=>
WR51X10055 Defrost Heater Assembly And location number 240 is the defrost thermostat=>
WR50X10068 Defrost ThermostatI would like to know what frost pattern and the true model number of your refrigerator so I can help you better. If it has the frosted over evaporator like in the photo above you can defrost with a hair drier and the refrigerator may cool again up to a week until you can replace the defrost heater and defrost thermostat. How to test these parts can be found on the same page as the part numbers I posted here, Thanks
Sea Breeze