There are many coaching certifications depending on what country you are in. You can think of them as a good start. None of them teach you even 1% of what you'll learn if you become a career coach. The best way to become a coach is to do just that...get a job coaching. You will learn as go.
When I hire a new coach, certifications are really not something that matter.
Things that are a must:
1. High School Education
2. Loves Gymnastics
3. Good with Kids
4. Good References (even if just former teachers and families that you have babysat for.)
Things that will get you a higher salary:
2. College Education
3. Gymnastics References
4. Management Experience
5. Sink or Swim Factor :scubadiver: (If you get past the first interview, I throw you on the floor with a preschool class. Will you teach my class...or will I have to save you after 3 minutes? How do you interact with kids? What's your personality like? Who is in charge of the class...you or them? "Oh...gee...wait...I'm not prepared" is the wrong thing to say, this tells me you don't have a "can do" attitude.