I understand it's just the BA lounge (which - as others said - is not great).
Gautrain is quick and easy - I would go to Sandton and hit on of the restaurants at Mandela Sq (e.g. Butcher Shop) or inside the shopping mall (Bukhara or Pigalle are my favorites).
While it can take a while to go through immigration on the way out (esp with a couple of flights arriving at the same time), I generally find things in the other direction (check-in, security) to be pretty quick so just try to get to the airport 1h30 before your flight.