This is not true. Assisted GPS doesn't rely on cell networks, it makes use of cell networks for faster fixes. They still work fine without service, but they do take much longer to get a fix. This is evidenced by the fact that you can put your phone in airplane mode and hold it near the window of an airliner and still get a 10-satellite fix.
Now, that said, you may be able to have maps downloaded for offline use, but in Google Maps searching and route-finding still require a data connection, limiting its usefulness.
Some people may have been confused on this point because Apple, up until iOS 8.3 (released less than 5 months ago), would also turn off iPhone GPS receivers when you put them into airplane mode.