We the problem is still there in 2019 which i find very strange.
however so far got it running by going in to the actual app - by right clicking show content and then taking out the app, strike suit zero and deleting the rest.
now trying to add the two dlc in t o it now but surely they should have looked at this, since 2016
but also it might help others
Support has this on the problem
Game "is damaged and can't be opened. You should move it to the Trash."
a) Redownload the game using the newest version of the GOG Downloader: https://secure.gog.com/downloader
To download, make sure the "GOG.com Downloader mode" is switched to ON before clicking the "Game installer" button. After the download is complete, reinstall the game.
b) Use this workaround if using the Download didn't help:
- Go to Applications -> Terminal
- Type in:
xattr -r -d com.apple.quarantine
but don't press Enter.
- Drag the application to the Terminal window, this should add the path.
- Press Enter.
it seems there is still no answer to that
I am on the same page, please someone give us an answer
They likely contacted Support and got an answer. Feel free to give it a try.
a) System Preferences -> Security & Privacy -> unlock to make changes -> select Mac App Store and identified developers.
b) Right-click (or cmd-click) the game -> Open -> confirm Open.
If that doesn't work go for option 1