GSAP 3.5.1 ( latest version ) is now included in Storyline 360

Yeah that is the position issue Owen mentioned before. Several fixes for that... if you refresh your browser after the change of size...well the positions get updated properly...but it might that thats not what you want.
Some of the other options are...
- using fixed elements in the Storyline for reference positions ( if you check my samples you find solutions for that )
- use vw (=viewportwidth) and/or vh (=viewportheight) for your animations. That is relative and thus can work properly.
- add a eventHandler for the browserchange.. JQuery has resize( ) for that... in Vanilla Javascript something like this works too..

var wrapper = document.getElementById('#wrapper');
window.addEventListener("resize", function(){
if(window.innerWidth < 768){
console.log('narrow');
}
else{
console.log('wide');
}
});

I attach a older sample that has some of the functionality needed in it. Not all, but with some puzzling and downloading some of my samples you should be able to figure it out. When time permits i mock up a new fully functional sample. At the moment no time alas.