DontGo It's mainly a case of being considerate of other people and not thinking that you own the place (which unfortunately is the message that some people are getting about OP from the initial post, though subsequent posts explain this is not the case) .
for example, if the gym is busy and all the treadmills are in use, don't spend an hour on them (gyms often have notices up suggesting a maximum time in busy periods).
Or, if the gym is busy, don't monopolise 4 bikes with you and your 3 friends gossiping about what Sue said to Linda, and don't actually do any/much cycling. And maybe, be aware that the gym is a public place and actually most of us are not interested in what Sue said to Linda anyway, so there is no need to have the conversation very loudly. Particularly if it's quite explicit (though my teen thinks it's exciting to go to the gym in January as he hears quite a lot about people's sex lives).
If you're using one of the weights machines, it's fine to sit on it while you catch your breath between lifts, prior to lifting some more. It's not fine to sit on it for 10 minutes, playing with your phone, while other people are waiting. You should get off and offer to alternate with another person if you genuinely want to keep using it but just want a longer break.
if the gym is not busy, it might ok to set up 3 or 4 machines/pieces of equipment and expect other people to leave them alone while you rotate round them (or to alternate with another person, as above). This is also not ok if the gym is busy (especially if other people are waiting for the things you're using). And when you've finished, put the equipment away, before you get the next thing out, so the next person to arrive doesn't spend 5 minutes searching for the dumbells to find they are under the kettlebell rack which is completely the opposite end of the gym (this happened to me yesterday).
If you're used to swimming, it's akin to the rules that you only swim in lanes if you are actually lane swimming (not messing about), that you swim in the lane that is appropriate for your swimming speed and that you swim in the same direction as everyone else in your lane.