Feshie is definitely the nearest airfield, its only a couple of miles out of Aviemore. Whether you get permision to use it however may be a problem. Like many gliding sites, the planning permission is possibly for
'gliding business only'. Aboyne is exactly the same, and is also a significant journey to Aviemore by car. Its the other side of the Cairngorms!
The nearest GA hubs, belive it or not, would be either Perth or Inverness. Both would be roughly an hour by car(or 40 mins by train) to Aviemore.
Unless you can get Feshie, your options are fairly limited. Try here -
Cairngorm Gliding Club for contact details for Feshie.