I have Stalker: Shadow of Chernobyl, but the 'Install' button is grey and I can't clock it, nor can I find a solution anywhere.
What am I doing wrong?
Cheers.

You mean the button in galaxy is grey. Just navigate where the game has been installed to and run the exe in there. Galaxy is optional and you do not need to use it.

You need to verify the files in GOG galaxy goto the game instead of hitting the install

If you are using galaxy 2 there is this button that looks like

-0----
----0-
↑this↑

click it scroll down the box until you see

Manage installation

then click it

then a list will come up

in that list you will see

Verify / Repair

Click that and wait for it to do it's thing then you should see the install button turn to a green PLAY button