There is a few ways and the best way depends on some factors.
Are you mixing game objects and gui? As in do you want to draw Game Object → Gui → Game Object?
If that is the case you wil have to change the render script. You can create a new material with a new tag, assign a render predicate and draw it in the correct place.
Do you want to draw this Gui scene behind another gui scene? Gui → Gui → Gui
gui.set_render_order is what you are looking for then
Are all your gui elements in the same scene?
You can then control which order they draw in with the heirachy (and layers)