Google Maps access the map data online everytime you open the app. Those data can be saved using OffMaps(US$2.99) from the App Store. It uses the technique of saving Google Maps data cache.
Here is another way to save the cache data. There is a new free app called Maps Enhancer from Cydia. Install it and set the level on how large the data cache should be saved in the Settings. While this is not possible to save the whole Google Map data on iPhone, both OffMaps and Maps Enhancer provides a good way to reduce the data transfer. More importantly, the map now loads quickly.
If you are looking for the completely offline map of your current location, try to search it on App Store. There are several apps that have the offline data of certain countries or places.
