I believe the success of the station that started it all for the oldies revolution for all CBS-FM stations in the country WCAU-FM under the genius Diamond Jim Nettleton had a library of over 2000 titles including future gold. I don't believe WCBS-FM included future gold when they signed on with oldies somewhat later, but they did have doo wop gold cuts every hour, neat.
If one wants to be picky, an oldies station with "future gold" is simply a gold-based AC. It's a hedged bet that some stations made way back then because they were not sure a station could thrive just on gold.
The gold format took quite a few turns, including those brushes with AC and the hesitancy to morph by adding more 70's and reducing the 50's and 60's songs.