From what I've gathered it's for manual transmissions, but can also be used in any other applications calling for a GL-3 fluid. Although I don't know much about the GL-3 specification to comment further, it is not a synthetic.
When looking for a replacement it seems difficult to find much in a 75w80. Redline offers their MTL, which states "This 70W80 GL-4 Gear Oil (SAE 5W30/10W30 engine oil viscosity)" I'm not exactly sure how it equals 5w30/10w30. I don't know if it is a good replacement for oe non-syn or not.