Ubuntu finally decided to list .deb packages on the app center.
Please consider adding this feature to Zorin. 
Attempting to uninstall OpenOffice and some other pre installed apps was way too difficult - it took me a while to even understand where these apps came from.
I'm a Windows refugee - like most people coming from Windows, I'm going to expect to find a complete list of apps and manage them from one screen. 
In Zorin .deb packages (Zorin apt packages) are already shown in the software store.
The new function shown in this article wouldn't help for .deb packages which are downloaded from developers sites and not from ubuntu or zorin repositories.
Zorin already integrates .deb in Gnome Software. the Article describes that Ubuntu's own App Store seems to support in a better Way .deb Versions. but this is more a Step back than forward because Ubuntu supported it in the Past.
It is referring to the Ubuntu Snap Store, which Zorin OS does not use.
Instead, Zorin OS 18 uses the Flatpak Store... But it has the option available to use the Standard APT packages instead of the alternative Flatpak.
I suspect the O.P. may have had Flatpak issues, rather than missing .deb packages.