It should be that the weather is not hot enough and your legs will be cold without pants, so you should wear pants outside your gymnastics suit. If you are in a warmer room, you usually only wear gymnastics clothes. You can wear a thin pantyhose on your legs at most to increase your beauty. You can wear pantyhose inside or outside your gym clothes. The form is also called "Shebin". You can check their clothes by looking up Shebin.