Uh yea so the is really old and I have honestly kind of moved on from unity since. But I’ll do my best to help you. If you just want to move the center point of the gizmo then do the matrix multiplication and then add an offset. The term for it is called a pivot, and that is the point where it will rotate around.
How I would do it with pure vertices is: take all the vertices and move them with -pivotpoint, then do your rotational math and then you would have them (hopefully) placed correctly.
If that’s a bit unclear then tell me and I’ll draw it up tomorrow on some paper.