IMO, there isn't a whole lot special about marine gear lube. Maybe some additional corrosion inhibitors in case water gets in, but I can't see a whole lot else being different. And if water gets in to any great degree, the lubricity will decline pretty quick in any case, so I'm not convinced the corrosion inhibitors would be of much use anyway.
I use West Marine branded gear oil rather than the pricier name brands, but wouldn't e averse to using automotive gear oil if I needed to. Maybe check/change more often (or maybe not), but it wouldn't keep me up nights.