I have never had this happen to me - to see any guy at the gym commando while working out. But I have had many guys stare at my crotch while working out. Even with a jock on I have a noticeable bulge. Sometimes while doing leg presses my jocked package will slip out one leg of my shorts.
In the locker room/showers/steam room/jacuzzi every guy in there stares openly at my dick. Sometimes a guy will look at it, smile and compliment me and I'm like "Oh, thanks". I have only once in my life ever seen in real life a guy better hung than me. In the showers, there was this guy with the biggest dick and biggest balls I have ever seen. It was amazing. I wasn't aroused in any way, but wow. To see something that unusual and impressive is great. He must have been nine inches soft and super thick and enormous balls. I thought "now why can't I be so fortunate?" oh well. Good for him.
But per the original post, if a guy looks and gives me a compliment I appreciate that. Happy cock spotting at the gym!