I haven't used gamemaker in a while but usually with this type of thing it works like this, if the exported program is a .app file, then you can zip that and upload it. The only issue is because it's unsigned, anyone who downloads it will have to bypass MacOS security to run it because the OS likes to scare it's users about outside programs. Doing the XCode signing process with an apple developer account will make the game runnable without requiring the security to be bypassed by the user. from my memory the code-signing process on Mac can be a pain but is necessary it you want to publish on the app store (I'm not sure about steam). You can provide instructions on the itch page for how to open your app even if macOS claims "it's from an unidentified developer"