Turn off your GTEL A7100 X3
Press and hold both Volume up and the Power button until it turns on.
Scroll to 'Wipe data/ factory restore' (scroll with Volume Down)
Confirm selection with the Power button
Scroll to 'Yes---delete all user data'
Again, confirm with the Power button
Wait until you see Wiping complete
Short press the Power button to 'Reboot now'.