The 30-year-old Gerhard collapsed while exercising on Monday, after suffering a massive aneurysm.
He was rushed to hospital and put in an induced coma.
He has bleeding on the brain.
Moaner, 64, played 17 Tests at lock for the Springboks between 1974 and 1980.
Gerhard is married to Janika and they have a one-year-old daughter, Lara.