Modifying the Palette’s layout is the correct action. But the Palette by default has a Diagram.layout that is an instance of GridLayout, which I would think would be more appropriate and more efficient than a TreeLayout.
So all you would need to do is:
let myPalette = G(go.Palette, div,
"layout.spacing": new go.Size(1, 1),
model: new go.GraphLinksModel(operators)