If it is not an outboard engine gear oil it won't contain the additional corrosion inhibitors to guard against the effects of water ingress.
If it is just, as I suspect, an automotive oil, then the objections to using it on this basis are just as valid as the objections to using automotive sae 80/90 oil.
Marine oil is a special hypoid oil, which will mix with a small amount of water and still maintain it's lubricating properties. Automotive oils won't do that.
Something to ponder!! GL5 is GL5 whether it is marine or auto. Water is a darned poor lubricant. Marine GL5 "supposedly" has "more" emulisifiers in it which allows water to mix with the lube should you have a leaky gear case. Since water doesn't lubricate well, why mix it with the lube that does. So either way, the gear case will not live long if not repaired. I've used auto GL5 (because I had it on the shelf) and because I did does not mean you "should". But my feeling is that in a properly sealed gear case, either will work just fine. Marine GL5 is yet another way to extract another buck or two from your wallet.