I really enjoy being tumescent in the shower and sauna at the gym.
I too am a grower, not a shower and I have found over the years that for me it is better to have to deal with semi hard ons and full erections that have my dick shrink smaller than I enjoy it being.
Here's how I deal with it. Have the shower water as warm as either you can stand it or as it will go. Focus on your dick and balls and the sensations they get being in the shower at all times. When soaping up, stand where the water hits your dick and balls, soap up your dick and balls often, before you soap up your body, after you soap up your body, don't be shy about stroking yourself once you've soaped yourself up. If you give your dick the attention it deserves, it will serve you well in the shower. What happens when I get a full hard on? Not much, this usually doesn't happen unless another guy in the shower raises wood. If I get semi hard---that's nice, I really enjoy being in that condition and it feels so good, especially with the hot water caressing it.
Next time you're at the gym and in the shower, stop worrying about it, relax and have fun, don't be afraid or ashamed of playing with yourself. That ought to work! Remember, there are other guys just like you, probably in the shower with you, what you're doing isn't going to freak them out, and it'll help them relax and enjoy themselves too. That way everybody can have fun in the shower, which is what we're looking to have, isn't it?