In order to hard reset a GFive U800, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
GFive U800 device will be brought back to the default configuration of a brand new device.
1. Turn on the phone by press-holding Power button.
2. Tap in this safety code - *#7370#
3. Enter the lock code, 12345 by default.
4. Now just wait for the smartphone to reset and restart.