Welcome to the forums, @brainybiz.
As far as GreenSock is concerned, we certainly don’t mind if you use the public tools (like TweenMax) in a theme as long as you’ve got the proper license (and if your theme is totally free, you wouldn't even need any special license from us). However, we can’t speak for WordPress or what they allow legally on their end. I’d recommend contacting them if you have questions about that. My understanding is that GPL is pretty tricky because it imposes some very strict (and in my opinion, burdensome) requirements that aren’t very business-friendly. I’m not a lawyer, so I very well could be wrong about this, but I believe the GPL infects everything it touches, meaning that if you use GPL software in your project, you MUST open source your entire project as well under GPL (which is why it can be very unattractive for businesses in competitive spaces).
According to https://www.gnu.org/licenses/gpl-faq.en.html#FSWithNFLibs it looks like it may not be compatible. GSAP is “free” in most cases (though of course NOT the bonus plugins) but since there’s a particular type of commercial usage that requires a paid license, it’s not 100% “free” in ALL cases. Again, none of these restrictions are imposed by us - if WordPress requires a GPL-compatible license, the restrictions are on their end. We encourage folks to use our tools as much as possible, including in themes. Over 8,000,000 sites use GSAP (and rising). We’re definitely not trying to be restrictive.
I hope that helps.
In case you haven't seen it already, I'd definitely recommend reading about the "why" behind our license here: https://greensock.com/why-license. We firmly believe our licensing model is ultimately a GOOD thing for end users like yourself.
Thanks for asking about this - I appreciate your desire to do the right thing when it comes to the licensing. Let us know if you need anything else.