Gravity isn’t metricized (and it would be hard to do so), the best we can do at the moment is indirectly measure it via +/- or other stats like what SC30’s teammates’ true shooting percentages are with him on and off the floor.
I think it's something that might be quantized through SportVU and the advanced analytics they provide, but not published anywhere to the public.
Second Spectrum has supplanted SportsVU for tracking, and is also the generator for all the advanced metrics that are now provided for the NBA as well as ESPN.