I can create 3d ball (spherical) shape with GIMP but now I need round surface that's curved inward. How can I do with GIMP?
This is a supplement for my comment, which said that the shading of the concave surface of a round paraboloid could as well be used for an elliptical egg. This ambiquity - convex or concave, the shading is the same - has been well known forever and one can easily find much more flashy examples of it.
Here's a construction example of a paraboloid lookalike:
In the right there's a flattened copy of the three layers of the paraboloid. It's the top layer in the layer list.
The layers are moved apart in the right. The concave surface really is like an elliptical egg - it's an elliptical selection filled with a radial gradient.
The edge is a vertical linear gradient in another elliptical selection which is so little off from the previous one that only a narrow edge is visible, most of it is hidden by the concave surface layer.
The backside of the reflector is a selection filled with a darker gradient. The selection was originally a circle, but a part of it was deleted to keep the left side of the shape behind the foreground part.
The brightness ranges of the gradients are all fine-tuned with the curves. It's difficult to guess in the fly the right gradients.
NOTE: The shading was made in the easiest way without any connection to the environment. If it must fit into the light conditions of a bigger painting, the task really starts to demand some talent and well sharpened drawing skills. Drawing it as an isolated item is possible for everyone.