According to this article on manners and common cense practice for fitness centers, “there are stated and unstated rules of etiquette and advice that, when followed, make the gym-going experience that much more enjoyable for you and your fellow gym members.”
The article points out that poor hygiene, blatant rudeness, and staring are examples of poor etiquette. That is a fair assessment. However, it cites improper form as bad etiquette. One probably should not classify that as poor etiquette. It also mentions “Slamming weights down and making excessive noise.” This is a bit of a grey area. Sometimes, people lifting really do need to grunt to get up heavy weights, and not focus on keeping quiet. At the same time, again with heavier weights, it is safer for the lifter to drop the weights on a floor with big bang than carefully try to maneuver them down quietly.
As a general rule, most of the norms in the gym are also norms out of the gym. The Golden Rule is not such bad thing to live by, unless, of course, it involves you throwing someone off their spin bike into a wall.