The Garratts of the classes GMA and GMAM are a development of the GM that was designed as a large and powerful locomotive for lines which only allowed a low axle load and had sharp curves. In comparison to their predecessor, they had the same power, but had roller bearings and improvements in mass balancing and other parts. They still had a mechanical stoker like the GM. Visually they could be distinguished by the rounded shape of their coal bunkers and water tanks.

The “M” in the GMAM only stood for “mainline” and meant that this class had larger water and coal supplies. Both classes were built to be easily converted into the other class, what was actually done depending on the demand. Some locomotives had also received an additional water tender. When production was finished in 1958, 55 had been built by Henschel, 33 by Beyer-Peacock and 32 by North British. With a total of 120, the classes GMA and GMAM combined formed the largest class of Garratt locomotives in the world.

The GMA's axle loading of less than 15 and a half tons ensured that they could be used on most lines, while the GMAM's larger supplies were optimal for longer mainline runs. They were also used on lines with a gradient of 3.33 percent. With the electrification and dieselisation of the SAR network, SAR could dispense with some. So 21 were leased to National Railways of Zimbabwe in 1979 and eight more were leased to Caminhos de Ferro de Moçambique in 1980. In 1983, nine were sold to the Randfontein Estates Gold Mines which operated them until the late nineties. SAR service already ended in 1984 and 11 were preserved, of which one is in operational condition.