Thanks @GreenSock - yes it is a bummer, and yes I am quite certain it is a config/setup issue, not at all suggesting it is a problem with the gsap package itself, but was hoping someone else had experienced this and had suggestions re webpack/tsconfig/some other magic bullet that would resolve it.
On the licence point, to clarify, I don't think I need one just yet, I am using the downloaded members package because I wanted to experiment with CustomEase. My project is still a long way from being releasable so is not commercialised yet (ie. end users aren't paying because there aren't any yet), and 'my developers' should probably read 'my future developers' - although I do have a freelancer help me from time to time so not sure if that counts, but I'm confused about this multi-developer point because I don't see reference to it on your licensing page.
(Repo may have been private, should work now.)