The two stations, which were put up for sale following a strategic review by GCap in November last year, broadcast in the North-West and North-East of England.
GCap will continue to sell national advertising for the stations for at least the next five years. Regional agency sales for Century North-West will also remain with GCap. The local sales teams will manage their own direct airtime.
The two stations have a combined audience of 1.2 million listeners and the deal gives GMG a total of seven regional FM stations in the UK.
Ralph Bernard, the chief executive of GCap, said: "This deal allows us to significantly reduce our debt levels and will also provide us with the financial flexibility to continue to invest in priority areas."
John Myers, the chief exec-utive of Guardian Media Group Radio, said: "This gives us increased scale and consol-idates our determination to be a major force in radio."