I wondered if anyone had any suggestions for making a quick and dirty debug GUI layer for LWJGL3?  Something like Gizmos in Unity where you could render lines, maybe points over top of your rendered image? 

Cheers!

Since LWJGL 3 is not a rendering engine but (merely) Java bindings to low-level libraries like OpenGL, Vulkan, etc., any such solution will obviously depend on your own layers of abstraction in your own application/engine. You can of course simply render a line with OpenGL or Vulkan in many different ways (targeting different versions of OpenGL or Vulkan).

Just use the LWJGL3 bindings for Nuklear or NanoVG.