Flip the v texture coordinate rather than the u.

Assets/Scripts/TestDepthImage.cs

             {
                 case ScreenOrientation.LandscapeRight:
                     rectSize = new Vector2(maxDimension, minDimension);
-                    rotation = 0.0f;
+                    rotation = 180.0f;
-                    rotation = 180.0f;
+                    rotation = 0.0f;
-                    rotation = 270.0f;
+                    rotation = 90.0f;
-                    rotation = 90.0f;
+                    rotation = 270.0f;
                     break;
             }
             m_RawImage.rectTransform.sizeDelta = rectSize;

Assets/Shaders/EnvironmentDepthGradient.shader

                 float cosrot = cos(angle);
                 float sinrot = sin(angle);
 
-                float2 tmp = float2(0.5f - v.texcoord.x, v.texcoord.y - 0.5f);
+                float2 tmp = float2(v.texcoord.x - 0.5f, 0.5f - v.texcoord.y);
 
                 v2f o;
                 o.position = TransformObjectToHClip(v.position);