You could try to scale it with a -1 value **[Entity.SetModelScale
http://wiki.garrysmod.com/favicon.ico](http://wiki.garrysmod.com/?title=Entity.SetModelScale)**, this would reverse all the triangles. The problem with this is it will reverse the order of the vertices, causing the engine to cull (not draw) the ones on the outside because they are now counter clockwise. This would make the inside vertices clockwise, since the engine doesn’t cull these, they would be visible.
The reason the engine does this is to optimize rendering. When triangles are facing the camera, their vertices are clockwise, when they are facing the oposite direction, they are counter clockwise. It is far cheaper to check if vertices are clockwise than to render the triangle, so it improves FPS and rastering time.
You would have to find a way to disable or reverse (make it cull clockwise vertices) culling for this.
Though, if you scale it negatively in only one direction I am not sure what would exactly happen, but I think it would still have culling issues.