I’m surprised by how many G-Shock owners don’t know this. It’s also a good way to sort out fake G-Shocks as most fakes do not have this function…
Most G-Shocks have function you can turn on and off where you can tilt your wrist slightly as if you are reading the time and the light will come on automatically and go off when you straighten your wrist.
You can consult your manual for “auto illumination”.