When I try to add a printer (HP 2420DN) through either System/Administration/Print Settings or CUPS, it finds the printer,
but can't find any printer drivers. I tried getting on Telegram, but to no avail. Has anyone else seen this behavior? Bug
or feature? Thx in advance.

Hi! I have just installed GhostBSD 19.04 on a new hard drive on my laptop and trying to add my HP2010 printer the same way you tried did not work either.

So I installed hplip package through System/Administration/Software Station; which brought the HP Device Manager application in Applications/Accessories.

In HP Device Manager the correct drivers were automatically selected and the printer was added properly.
Checking in System/Administration/Print Settings afterwards, the HP printer icon had appeared (it was not there the first time).

Printer been tested and working nicely
HPLIP gives you a nice graphical toolbox and enables you to check your cartridges' ink levels, do the maintenance of your printer and scan through that interface if your HP printer is having one.

As another example: the driver package I need for my printers is called "foo2zjs"

good luck!

I have checked and the package foo2zjs is available in Software station.

Some printers use hpijs and Software station has the package foomatic-db-hpijs available

HP LaserJet 2420dn Printer is on the list of HPLIP's All Supported Printer Models, with full support but end of support.

It seems to be using hpijs driver, which is an add-on printer driver for Ghostscript.

It is listed on Open Printing site as working with hplip, you might find this helpful:
http://www.openprinting.org/printer/HP/HP-LaserJet_2420