GMP is a "new?" economic term I would imagine much like GDP is for nations. Gross Metropolitan Product I assume much like GDP conveys how much "stuff" your metro area produces / consumes in a given year. well for the big 3 SC metro areas Columbia leads the way followed by Charleston and then Greenville.
Columbia - $30.4 Billion GMP
Charleston - $26.3 Billion
Greenville - $24.8 Billion
In fact Columbia, SC has a slightly less GMP then Kenya does GDP and slightly more than Lebanon's GDP.
see pdf chart presentation below for more info
http://www.usmayors.org/metroeconomies/0110/charts.pdf