Earlier quoted context omitted.
Google does their best to make the app unusable off-line, but I hear there are alternatives (HERE Maps is a name I recall seeing on HN a few times).
Google Maps for Android provides area-based offline maps, with a time limit that is not enforced. It's been very useful to me personally, not sure what's unusable about it. I find the online version less usable because it nags me about GPS and obstinately only stores search history online, coupling it to the global Google Activity History setting.
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.