A question regarding GUI

Hello,

Sorry if this is in wrong place or otherwise an unclear question, but here goes: Is it possible to create a GUI in another program (Such as visual studio) and then proceed to use it inside unity either as a customized editor looks or as a playmode UI as if it was a stand alone tool?

For example, what I am trying to achieve is moving from this to something like this (Source picture for the second one.)

EDIT: I am attempting to create various tools for my game project, currently working on map editor.

 

It's not possible in the way you describe it. You can certainly create your own tools, but they'd have to be written using Unity's GUI constructs, not imported from another program.

The official tutorials have a section for Extending the Unity Editor that may be useful for you.