I was trying to fly indoor, but it has gps lock from 8 satellites which doesn't force to use OPTI mode.  So while starting the motors, it just sit there because of object avoidance kick off not allowing to take off.  How do you disable just the GPS?  I guess not only will need ATTI mode, but mode that disable GPS, which allowed OPTI mode to be effective.

Obstacle avoidance doesn't have anything to do with GPS and shouldn't prevent you from taking off.
There is no way to switch to ATTI mode.

Why would you not want to use GPS if you could?

Because the GPS location is right outside of the house, it will cause a crash if mavic decide to get to that GPS location.

Not sure what you're talking about.  I have used the Mavic indoors fine with and without GPS.  Just turn off the front obstacle avoidance sensors.

To force ATTI or lower GPS counts, place foil on the top of the mavic. Not sure where the GPS is located in the Mavic, but this is what p2/p3 used to do.
And make sure you set to HOVER for RTH. If by chance the mavic creates a false homepoint and the controller decides to wig out, the mavic may ascend to the roof and attempt to RTH.

QuanthonyTrang Posted at 2016-12-8 23:27
To force ATTI or lower GPS counts, place foil on the top of the mavic. Not sure where the GPS is located in the Mavic, but this is what p2/p3 used to do.
And make sure you set to HOVER for RTH. If by chance the mavic creates a false homepoint and the controller decides to wig out, the mavic may ascend to the roof and attempt to RTH.

GPS is in the tail of the drone. If you pop of the rear LED indicator cover, you will see it inside. It is the silver perforated module in the rear of the picture. You can see the lead wires for the antenna on the right hand side:

GPS has nothing to do with indoor flying. there was some bugs initially with ATTI switch to GPS and vice versa caused crash but that got fixed ..