I just found a solution. I don't have Gionee S6 but my friend has one.
Follow steps in order:
1> Reboot the phone .(by holding power button and tapping on Reboot)
2> When phone shut down and again power on , You will see Gionee Logo Flash Screen. At that moment hold the lower Volume button
3> You will be redirected to Factory reset mode.
4> You can move down the cursor using Volume down key only.... Try to highlight the "Start Test"
5> Once you highlight the "Start Test", Press the power button and your test will be start.
6> Allow the phone to complete all test...
7> Ignore the Failed test
8> Now reboot the phone
Congrats you will see Both Sim is working perfectly....!
Please let me know if you find any difficulty in above step.