Is there any tutorial for how to rigg bipeds in gmax? Is it even possible to rigg bipeds in gmax?
I need it in order to make some good cutscenes for my maps :)

IK Limb Solvers. They're a little like strings on a puppet. Look it up in gMax's reference. You'll want 2 per limb, from foot to upper leg, and from lower leg to pelvis, then from hand to upper arm, and lower arm to shoulder. You may even want to apply them further with head, neck, and spine.
Then look up constraints. These, and the limb solvers, are helper objects. You animate them, and they animate the nodes of the character, or vehicle, device machine, etc.
Finally, you'll need to learn how to "collapse transforms". This will give the nodes keyframes, so you can delete the helpers and export the jm_ animation.