J Atkinson's post in Uncaught TypeError: Cannot read property 'length' of undefined in WordPress was marked as the answer
I've found the answer!
As it turns out a plugin (LayerSlider WP) I believe also uses Greensock, so WordPress was loading it twice.
I deactivated LayerSlider and no more errors!!!
Hopefully this will help anyone else using GSAP with WordPress.
By the way, thanks for the great support.