Plus codes are basically a translation of a lat/long coordinate into something that can more easily be communicated between humans. Code to do so is available via Github (link in the description of this group), and works without having to access Google's Geolocation API.
Caveat: there are two different variants of plus codes. The first is a "full" code which is globally unique and works as described above (format: "CFGHXXXX+XX"). The other one replaces the first four characters with a reference to a city or other location (format: "XXXX+XX, Place_Name"), and can only be translated back into lat/long if you have the proper context (that is, position of the reference location).
I don't actually know if the Geolocation API offers this functionality out of the box - but some sort of location lookup will likely be necessary for the second variant to work in your usecase.