Hey I ran into this roadblock earlier today. I don't know if you ever got it working, but I finally did a few hours ago and decided to share my success here just in case you or anyone else stumbles across this post and needs a little assistance.
Here is the link to my sandbox I created https://codesandbox.io/s/react-scrollmagic-gsap-8mp14
Doc used for react-scrollmagic: https://www.npmjs.com/package/react-scrollmagic