This is a really old demo from 2014 when I really started getting into 3D with GameMaker. Since I found it, I thought I'd post it! I think I've come a long way, eh? At this point I was using Model Creator by Marteen Beart to make all the models, then manually offsetting all the models to replicate bones. I think it shows though... These days, I stick mainly with Blender.
It wasn't very well optimised, but it worked. I was using a basic implementation of Inverse Kinematics constrained to two axis. Everything the moves actually used a bit of IK, broken down into chains of 2 "bones". The legs, arms, spine and head all had a chain, with hands and feet added on with extra rotation, and a bit of extra twist for the head.
If you look at the motion of the hands, you can see them make circles. The same is true of the feet (though more elliptical), but they we're clamped above z=0. I just made use of sin and cos with time a lot for this one.
I'm looking into adding IK to MaddeModelKit at some point, and it'll probably be somewhat based on this original implementaton. I imagine it's gonna be much better, given the 4 extra years of experience!
It's kind of fun to imagine that I was into 3D animations and interpolation and stuff in GameMaker way before I dove right in. The head even turned to points of interest!