I will test your solution tomorrow, in order to make it more 'general'.

I just succeeded in making the gizmo rotate in sync with the joint by using local angles.
But, the local angle of the gizmo is the same as the absolute angle, so I keep track of the local angle in skills.

When I rotate a bone, I rotate the absolute angle of the child bone gizmo to match the new "absolute" orientation of a bone, but it still keeps the local angles intact in the gizmo skills, and when I need to rotate the child gizmo and bone I use the difference between the old local angle and the new rotated local angle and rotate the bone.

Currently, my solution isnt good, because (out of desperation) I've defined all the bone positions, orientations, handles and I use a separate function for each gizmo at every joint. I currently have two joints with gizmos testing and I'm already at 300 lines of code for the gizmos alone

I intend to share the script, so I'll have to make it more general, without hard definitions...

I'll tell you how it goes...