Neither GMax nor NifSkope are designed for animation. GMax is essentially 3dsMax, minus animation (and a couple other things). NifSkope is meant to be a low-level editor, and doing animation is possible but would take forever.
If you want to animate stuff (and you're probably better off doing it yourself, both to stay true to your vision and it'll be faster), you'll need a full modeling program. If you can get Max 5, you're set. Unfortunately they don't make or sell it anymore, so you'd have to run across a copy online or in a store somewhere you can pick up. Otherwise, the more recent copies of Max work ok and people have reported Blender can export NIFs, if you can figure out how to work the thing.