If a police officer is stopped at a red light, and my vehicle was traveling from behind, how far away will the rear radar gun be able to ?

A lot further than you think. I drive a 2019 model semi truck for a living and just bought a 2020 car for myself. Both have radar, sonar, and adaptive cruise. The radar on the truck tells me how fast a vehicle up to 650 feet in front of me is going. I'm not sure how far it is exactly in the car but it's about the same if not further.

Police may have more advanced capabilities. Like their radars might be able to tell a lot more things than just speed and they may be able to do it as long as they can see and be aimed at you.

The accurate range of the radar is significantly more than the accurate range of your eyes. In other words, if you can see the patrol car, you are WELL within range.

Radar guns are accurate for a very long distance. How long depends on many factors. The biggest factor is how many other cars are in the area and how close they are to you. If there are no other cars nearby, If I can see you I probably have your speed. 

You got the answers below, plus many cities have plate readers on the patrol cars and they read your plate and notify the police officer if the car is stolen, you have a warrant, suspended license and or expired plates and other goodies

On a straightaway, thousand of feet.

At least 100 yards.

Up to a mile on clear roads. Laser radar guns are very accurate.