OK, I should have googled it first. If anyone else gets the same problem, try this: https://www.minicreo.com/mac-uninstaller/app-cannot-be-opened-on-macos-catalina.html
Can't install GnuPG-2.2.20.dmg on Catalina
Somebody sent me a request for such a package yesterday, but my reply bounced. If...
I can provide a Debian package for grip (3.3.1, anyway) if anybody's interested....