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The program was very interesting from both a historical and experimental viewpoint. The GP-B was designed to confirm two key predictions of Einstein's general theory of relativity, now some 90 year's old. It wasn't until recently that the technology became available to actually carry out the experiment. The 1.5 inch gyroscopic spheres used, if blown to the size of the Earth would have no peaks higher than 2.4 metres. They are the roundest objects ever made.
Stanford University Professor Francis Everitt spent 40 years of his life getting this project to reality. What struck me was the continual struggle by Everitt to keep the project going, as funding was cancelled 7 times. The Gravity Probe B was finally placed in orbit around the Earth in 2004. A lot of initial calibration was necessary. The geodesic effect was confirmed but frame dragging needed more data. NASA funding evaporated in 2008 but the project was kept going by a Saudi Arab to get results for frame dragging.
From my understanding, the proof for the geodesic effect was strong but the evidence for frame dragging is still to be generally agreed upon by the scientific community.
Regards, Rob
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