There are many reason why you want to go to the gym. The reasons might be because you want to lose weight, you want to be physical active or you want to build some muscles. Regardless of the reason, you need to wear activewear pants and t shirts to the gym. That is the ethics or policy of the gym. You wouldn’t be allowed to use any of the equipment if you are not dressed in these.
I also agree with this policy. For instance, if you were allowed to wear jeans to the gym, you would not have much allowance for movement. You would be restricted by your jeans clothing. But with the activewears, you would have free movement. The only thing that would restrict you now is your body i.e you have reached the limit your body permits you to reach. This is because the activewear pants and shirts are made from nylon (not all are 100% nylon. They are fabricated with some other materials).
Also, wear activewear clothing would help your sweat to dissipate. As we all know, if you are doing a strenuous activity, you would excrete sweat. And if the sweat cannot be dissipated, you would stick and feel uncomfortable. But if you are wearing the activewear clothing, you would be able to expel the secretion because the clothing is airy.
So with this, hope you see the reason why you need to wear these type of clothing. Basically, it is for your own safety, and the gym is trying to also protect you. Also, you shouldn’t forget to buy the right sports footwear for your feet. It is usually preferably to buy classic sneakers because they would last longer and be comfortable for your feet.