On page 16 of the GAO report it says, "The critical design review is the time in a projectís life cycle when the integrity of the productís design and its ability to meet mission requirements are assessed, and it is important that a project ís design is stable enough to warrant continuation with design and fabrication. SpaceXís final planned design review was held in August 2016; however, the program reported that a number of outstanding areas, primarily related to ground systems, still needed to be reviewed. SpaceX officials told us these areas were reviewed in November 2016."
So, the critical design review is complete. I'm not sure exactly which milestone this refers to, but it sounds like a significant development.