Here are the parts, your model number seems to be missing a couple digits at the end.
Replacement parts for GLRS264ZA models | AppliancePartsPros.com

Is the compressor constantly running?
You can usually hear it but you should also be able to feel it vibrating and it should be warm.

Is the evaporator (freezer) fan running?
It is required for proper cooling.

Is the condenser (compressor) fan running and are the condenser coils clean?

If the compressor runs constantly and both fans run then remove the cover inside the freezer so you can inspect the evaporator coils.

If they have frost in only one area, usually where the freon enters the coils then you have a sealed system problem.
Be sure the compressor runs for a couple hours because if you do this check just after a defrost cycle you could jump to the wrong conclusion.
A sealed system problem is an expensive repair so you do not want to make a mistake.

If they are heavily iced/frosted over then it is a defrost problem.