Jack didn’t initially create Greensock to be a monetized business, but rather a helpful tool for other developers. He was working at an ad firm doing animation work, and it was there that he found the inspiration to create Greensock.
The Robert Penner easing equations were like magic for Jack. Such simple equations that could create such cool effects were terrific. He talks about how he doesn't consider himself a math wiz in the slightest. The visual feedback that animation gives with the equations however really help him solidify the concepts.
Jack's success turning his side project into a successful business is genuinely impressive. It was a stressful time for him; he was working for the agency still at the time. People don't feel safe with a product that might lose support any day, so he goes into the security behind a financially backed product and the factors that lead to him monetizing his business.
Finally, there is a lengthy discussion on the challenges of having to maintain such a universal library that works on nearly every platform. There are a lot of edge cases. Jack also makes an effort to keep the API stable, so that questions that you find online from ten years ago can still apply to you today.