@Sam R I just tried this on beta 3; and got "Invalid modes 'private_data:on'.
I see reports that it stopped working with beta 2. I'll look for more info.
@Sam R My DMG opened in b3; however I didn't a Gate Keeper dialog at all. I've even re-downloaded it; so I can only assume that the OS has tagged somewhere that I've manually opened this DMG in the past.
When you say "gatekeeper dialog" - do you mean the dialog that says "verifying..." and has a progress bar? Or some other dialog.
If it is remembering which DMGs you've opened, I wonder where it's stored and if there's a way to reset that?
Edit to add: I just ran a test using the latest Xojo alpha (which is notarized). On both 10.14 and 10.15 (beta 3), when I double click the DMG I get two dialog windows, first "Verifying..." and then "Opening." After ejecting the DMG and repeating it, I see pretty much the same thing: on both 10.14 and 10.15 there are again two windows (although the "opening..." one seems faster). So I'm not seeing evidence that the OS is caching or remembering the DMG and bypassing any checks on a second open.