The one mentioned is a Premier Club lounge - used by BA and accessible to holders of Diners Club cards among others. (I am baffled by wideman's experience - the BA web site specifically states that this is available for J pax.) You can also pay to enter - I think the fee is R106. Rather a nice lounge actually - very airy and with lots of light from windows.

SAA also has an arrivals lounge on the mezzanine level as you exit customs. There are indeed showers. Not sure if it's designated a *G lounge.