how does the speed of visible light compare to the speed of gamma rays, when both speeds are measured in vacuum

The speed of visible light is equal to the speed of gamma rays.

Both visible light and gamma rays travel through a vacuum with the same speed which is about 3.0 x 10⁸ meters per second. Both visible light and gamma rays are electromagnetic waves which travel through space with the speed of light which is 3.0 x 10⁸ m/s. The speed of light is the fastest speed possible, nothing travels faster than the speed of light.