The real reason you are not supposed to shower with them on is because of the water heat. Heat causes materials to expand at different rates, so when you waterproof test a watch, that rating would not apply at the elevated temperature of shower hot water. Every manufacturer has this warning in their onwers manuals, so I highly doubt it’s based on nothing. I suspect it’s based on denied OEM warranty claims. Proceed at your own risk. The other funky thing about water, when it gets inside it doesn’t necessarily show right away. So you might be allowing small amounts in that eventually will wreck the internals, but you think it’s all fine right now.
The more interesting question, why do you need a watch on in the shower? Is it that hard to manage shower time without one?
If I HAD to wear one in the shower, I wouldn’t care that much about a gshock, they are low price tool watches that are easy to replace anyway. If I wreck one, it’s not the end of the world. But anything more substantial (expensive), I’m not taking the risk.