Gray screens suck when nothing works and your poor

Considering the age of the device, it may be a hardware failure. Take it to your nearest Apple store for diagnostic tests (that is free) and then you will find out what the problem is and if it can be fixed.

Read my reply in this Discussion: Laptop keeps restarting even before and after clean install. Your Mac may be suffering from the same problem.

Apple will no longer service it, but there are non-authorized repair options.

a non-trivial number of 15-in Dual Graphics computers suffered from the failure of the discrete graphics chip. If you are NOT using an external display, you may be able to use a hobbyist hack to disable the Discrete graphics processor and continue with only the Integrated graphics processor. If you want to edit Videos or use external display, these hacks will not do anything good for you.

The easiest one to install requires that you boot into single-user mode: Hold command + S at startup. if you get to a black terminal screen with white letters, there is some hope you can apply the patches required.

This hack is not for normal use, you may hurt yourself or you computer, and novice users should not even consider it.

There is one more than John mentioned, that was developed as a side effect of pushing those old, unqualified Macs into later MacOS than Apple supports, and that is why I am being circumspect. I have been scolded for encouraging novice readers to take excessive risks.

If your Mac can boot into Single-User made and you want to proceed, I can provide some stronger hints about how to find that hack. Post back if you are willing to assume those risks