Would be nice to know that F9 closed the game(if this is common in older games, I am not familiar), even if only a note in the status menu or main menu screen. Having to search the internet to simply close the game seems unnecessary( I know how to force close in task manager/ or right click icon on status bar, but not everyone is tech savy)

It is not common :)

While playing, F9 brings you back to the Main Menu.
In the Main Menu, F9 closes the Game.

You can also directly ALT +F4 if you want to close the Game.

But yes, a little Manual would have been nice, alone because i love those Extras in other Games :)


Edit:
In many older Gams F5 ist Quick Save and F9 is Quick Load.

Yes GOG!?!? Where is the actual MANUAL for bonus content? Even tho... I read it a while ago, and I don't think it mentions how to close the game either. LOL
EDIT: The manual lists the F9 key as Pause Game / Abort Game. It does not explicitly state that pressing F9 also lets you quit the game. But abort is just another word for quitting.

Thanks F9 can be use to close the game, I know sound dumb but yes GOG should add some manuals or text to mention this shortcuts. I mean thanks GOG to bring this gem but make it more love to the work

You don't have to be tech savy in order to try out the F buttons from 1 to 12, and you also don't have to be tech savy in order to do a quick Google search in the age of the internet. If someone doesn't know how to quit, they can just try out the buttons, or quit with ALT+f4, and than do a quick Google search. Problem solved.

While most physical releases might have an advertising insert, if anything, Japanese ones often include quick reference cards to explain button functions, even today. I think every game release should have something like that, but that's just wishful thinking at this point.