Unity Script Reference:

OnPreviewGUI(r: Rect, background: GUIStyle): void;

void OnPreviewGUI(Rect r, GUIStyle background);

def OnPreviewGUI(r as Rect, background as GUIStyle) as void

Parameters

r	Rectangle in which to draw the preview.
background	Background image.

Description

Implement to create your own custom preview for the preview area of the inspector, primary editor headers and the object selector.

If you implement OnInteractivePreviewGUI then this will only be called for non-interactive custom previews. The overidden method should use the rectangle passed in and render a preview of the asset into it. The default implementation is a no-op.

Note: Inspector previews are limited to the primary editor of persistent objects (assets), e.g., GameObjectInspector, MaterialEditor, TextureInspector. This means that it is currently not possible for a component to have its own inspector preview.

using UnityEngine;
using UnityEditor;

public class GameObjectEditorWindow: EditorWindow {
    
    GameObject gameObject;
    Editor gameObjectEditor;
    
    [MenuItem("Window/GameObject Editor")]
    static void ShowWindow() {
        GetWindow<GameObjectEditorWindow>("GameObject Editor");    
    }
    
    void OnGUI() {
        gameObject = (GameObject) EditorGUILayout.ObjectField(gameObject, typeof(GameObject), true);
        
        if (gameObject != null) {
            if (gameObjectEditor == null)
                gameObjectEditor = Editor.CreateEditor(gameObject);
            
            gameObjectEditor.OnPreviewGUI(GUILayoutUtility.GetRect(500, 500), EditorStyles.whiteLabel);
        }
    }
}