Forcing a GameObject to the highest depth (closest to camera)

I am Instantiating my own gizmo Prefab at runtime, when clicking on a GameObject.

I am using the selected objects transform.position and transform.rotation to place the gizmo in the center of the GameObject.

This is at runtime, not in the Editor. So the built in gizmo functions aren't available.

The Problem:

I would like the Gizmo to appear on top of everything else, but maintaining its proper position in relation to the GameObject.

Basically, what the Unity Editor does with its Gizmos.

Is this doable?

See this question:

Rendering Order

I would like the Gizmo to appear on top of everything else, but maintaining its proper position in relation to the GameObject.

I don't understand what you mean by this... you need to be more clear.

Unity treats any gizmos that you draw yourself identically to those it uses internally. Unless you're doing something different (or you want to produce a different result), you shouldn't be having any problems. There's no difference between a gizmo you place, and a gizmo Unity places (by default).