free fall as in one of those planes that take a sudden dive so that the people in it and the observer appear to counter the effect of gravity so GP-b with it's suspended gyro's in effect has the characteristics of zero gravity and thus is able to make measurements of the warp effects on space time by a mass object without factoring in the effects of gravity...
...kinda like taking measurements in a plane dive that keeps diving
I am so out of my league here but you should see me when i care. I look exactly the same, just words on a screen