If the game's page in the store says it's Windows 8 compatible, that means the GOG staff managed to run it on their Win 8 testing machines.
The problem is that every computer consists of a lot of different components, drivers and software, meaning that any given game might not run flawlessly on your particular machine, even if it fulfills the system requirements.

A lot of people reported constant crashes with the game, but GOG apparently managed to find a solution for that. You can read about it here.

Your best bet is to just try it. If it fails, you can still contact support , and if they're unable to find a solution, you're eligible to get your money back, as per their 30 day refund policy.

So, to re-iterate for the last time and to make sure no misunderstandings are around.

I should:

1. Go to the Library.
2.Get the game.
3:Preferably install it with the (Discontinued?) Gogdownloader
4:Install it.
5: You know what comes after this.

If it fails to work regardless, either import game to another PC (Maybe using a disc?), or contact support. If no solution, sell and re-buy once the opportunity reveals itself.

So my advice to you would be this:

! If you don`t fulfill the minimum system requirements don`t download it and ask for a refund. ! (But I don`t think this is the case here.)

1. Download the game just with your browser (It is a small one so there should be no problem. I downloaded all my games only with my browser and never got a problem.)
Click on Account (upper right corner of this site), then library, now click on the game you want to download. A pop-up opens. Download the files under Game Downloads (left side) by clicking on them (some games are splitted in more files). The game should only be one exe-file (it is a small one). After you downloaded the game double-click the exe-file to install it.
2. See if it works. If you got problems write to the support within 30 days after you bought it. If you want to test it on another PC just copy the file on an usb-stick for example.
3. If they can`t help you and you fulfill the minimum system requirements you will get your money back.

Of course you could use the GOG-Downloader (https://www.gog.com/downloader) to download the game if you want (but there is no need to do so). The link you need for the download with it can be found by clicking on the more button.