Status: Implemented, and the update with it V1.03 has been released (http://www.neolemmix.com/old/gstool.zip).
Right now when creating animated objects, there's no easy way to check if they looks right other than saving the tileset, copying the tileset to the right location, opening a level and testing the object, then making small alterations and repeat. This is quite awkward and it would streamline object creation quite a bit if you could just see how it will look in-game animated in gstool itself.
I'll look into this one later today. :)