Because Pinegrow is a “standard” editor with visual features, just like with any javascript thing, you can use GSAP with Pinegrow since day one. With the help of GSAP documentation, you can edit your custom scripts from within Pinegrow with the internal editor or with an external editor like Atom or VSC.
However, because the Pinegrow Interactions add-on is powered by the GSAP engine BUT is “not” GSAP, you will need to purchase the Interactions add-on if you want to be able to use the Interactions features available in Pinegrow. (the visual timeline and so on > See our documentation).
You can read some interesting feedback about the add-on from satisfied customers (including from @Pervasive 
) here → Short impression of the amazing interactions plugin
The Interactions add-on was released on November 27 and received its first update on December 10.