You will need to start with a female character, be in good physical and athletic condition, and enter high school gymnastics, or college.
And this can be done from the activities menu, so you have to go to the gym, to do general athletics and martial arts, until you get to middle or high school, to learn gymnastics.
You can practice on the gymnastics equipment at school or as a leisure activity, in the school activities menu.
If it is not listed, you have to try again.
Also when you get to college, you can practice on the gymnastics team, which will allow you to improve at it, and also tap at your school and then gymnastics if you are a team member.
After introducing you to this article on how to practice gymnastics in Bitlife, we hope you can practice and be a member of the team so that your character develops and you can have a good level of health in the game.