To the best of my knowledge, currently three stations. From Wikipedia (which was originally authored by me, anyway):
A two-hour version of The Best of GSN Radio is aired on Sundays on WNYM in the New York metropolitan area from 12 p.m. - 2 p.m. Eastern and on CKTP in Fredericton, New Brunswick from 2 p.m. - 4 p.m. Atlantic. A one-hour version airs on KMVN in Los Angeles from 6 a.m. - 7 a.m. Pacific. WNYM and KMVN began airing GSN Radio in November 2008, while CKTP began broadcasting the program in February 2009.
CKTP also airs the last hour of GSN Radio live weekdays from 3 p.m. - 4 p.m. Atlantic.