With the launch of macOS Catalina (10.15), Apple now require you to notarize all apps that are not being distributed though the App Store. For more information on this, please see the Apple Developer article here:
For GameMaker Studio 2 games, the notarization process requires an Xcode archive, and as such is only possible for those games built using the YYC (future updates to the IDE and Runner will permit VM builds to be notarized). Note too that you don't need to do anything within GMS2 (other than build using the YYC) as the notarizing process is handled for you by Xcode.
To notarize your app for disribution outside of the App Store you need to follow these steps:
- Build your game using the YYC in GameMaker Studio 2
- In Xcode, once the build has finished, open the Organizer window.
- In the Archives tab, select the archive you created.
- Click Distribute App to view the distribution options.
- Choose Developer ID for your method of distribution.
- Click Next.
- Choose Upload to send your archive to the Apple notary service.
- Click Next.
When you click Next, Xcode uploads your archive to the notary service. When the upload is complete, the notary service begins the scanning process, which usually takes less than an hour. When the notarization process finishes, Xcode downloads the ticket and staples it to your archive. At that point, you need export your archive again to create a distributable version of your software that includes the notary ticket.
For full details on this process, please see the following Apple Xcode article: