Apple - Forcing `open` command to open unsigned app?

1. Create and add a Gatekeeper label to the app. (‘Approved’ is an arbitrary string.)

   spctl --add --label "Approved" /path/to/xyz.app
   

2. Approve all apps with the label.

   spctl --enable --label "Approved"
   

   This only needs to be done once and adding the same named label to apps in the future automatically enables their access.

3. Open the app as usual.

   open xyz.app
   

For more information about spctl (SecAssessment system policy security), see the man page.

You could strip the quarantine from the app:

xattr -r -d com.apple.quarantine /path/to/xyz.app

You may want/need to use sudo for that command, depending on permissions.

Then Gatekeeper isn't involved, at all. Of course, that also means that the signature isn't verified, which removes an important safety feature.