When I read "offline GPS" I assumed accelerometer/gyro/compass positioning, now that would have been cool. Some point and shoot cameras have this for tracking position when GPS/GLONASS signal is unavailable.
Just using accelerometer, as some point and shoots do, is pretty inaccurate but still somewhat usable. Using all the sensosrs on a modern phone (including signal strength from known wifi/tower beacons) would yield much better results.
My point is more in the spirit of: I got excited because there's little or no such apps on playstore, offline maps aren't anything new.