[vetinari]

But only in the Google Maps app. Nowhere else.

If you have a GIS system, that is able to use Google Maps as the basemap, you still don't have the ability to save it for offline use. The APIs/libraries/license agreement with Google that these GIS systems use won't allow that.

Not that other providers (Bing, Here, etc) are any better. Your only way is to download OSM data/obtain local ortophoto and make your own tiles.