How does the speed of visible light compare with the speed of gamma rays

They are the same

Explanation:

Electromagnetic waves consist of perpendicular oscillations of electric and magnetic field, which oscillate perpendicularly to the direction of motion of the wave (transverse wave). One property of the electromagnetic waves is that they travel in a vacuum always at the same speed, called speed of light:

Electromagnetic waves are classified into 7 different types according to their frequency; from highest to lowest frequency, we have:

Gamma rays

X-rays

Ultraviolet

Visible light

Infrared

Microwaves

Radio waves

We see that both visible light and gamma rays are electromagnetic waves, so they both travel in a vacuum at the same speed, the speed of light.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it