Thanks to @Schwim@lemmy.zip for the solution. The solution can be found in this reddit post. (click here to use a front end to view the reddit post)

Solution

To verify that a game is from the official GOG store and has not been tampered with, you can follow these steps:

1. Right-click on the game and click properties
2. Click on Digital signature

3. Check if it says GOG Sp. z o.o. (note that the amounts of dots in the name might be different from older and newer releases) If you want the fast and less safe option your done else go to step 4.

4. Click on Details and then Advanced
5. Click on the numbers marked with red

6. Copy the value

7. Open the GitHub link https://github.com/hippie68/gogcheck/blob/master/gogcheck and search for the copied value (Ctrl + F) If you get a match, the software is signed by GOG and has not been tampered with.