I have a GSS25LGPA side-by-side that the freezer completly thaws then freezes back a day or so later, then 3-4 weeks later it does it again.

Could be 1 of 2 problems ...there is a defrost terminator located on the coil itself that when the timer gets to the stage it checks for temperature and if cold enough it will go into defrost cycle. this could be bad.... 2nd it could actually be the defrost timer ...if this unit is 2001 or newer most likely has a pd board that works the defrost cycle and compressor operation...the board itself is bad and needs to be replaced... some models still have the old style defrost timer with gears in them either located on bottom of unit in front or inside the fresh food refrigerator compartment up top near the lighting assembly.....hope this helps fairly easy to repair....