The light is coming from left, the whole left frame of the sphere must be more luminous. If the effect isn't emphasized enough, paint it two or three times with the same method. You can also distort the cursor with the mouse on the button of the image and then place the center of the reflexion visually. The colours of this points are a matter of taste. The work is almost finished.
All creators have their own view of the objects and their own style.
Everyone will have a different opinion. Everyone should be amused. Because Linux, and more Gimp, or evenmore the enthusiasm of the creator should not be neglected as a source of amusement.