Answer I found on a forum:-
"Open Terminal or iTerm and type "chmod -R 755 " and drag the .app into the window, which will bring the full path into Terminal or iTerm.
It will look like this:
chmod -R 755 Path\ to\ app\ file.app
Press return
Then, you can open the app file normally, but it will fail because gatekeeper won't be able to verify the file. From there, go into your "Security and Privacy" in "System Preferences" and then click to allow the app to open."