What's a G?

I absolutely love this commercial. I know it's about Gatorade but it's just inspirational to me because of the words said and the famous athletes shown. Some of those figures have inspired me to keep going and fight for what I want. I can truly say people who have told me I'm too short or that I would never make it have only fueled my fire. They gave me more of a reason to keep proving them wrong. What is a G? I'm a G!