The 120 Class GMA/GMAM Garratts built between 1954 and 1957 were SAR's final 3' 6" gauge steam locomotives and the largest numerically of all the Garratt classes.An improvement of the earlier GM Garratts, they had modern features in common with the Class 25 4-8-4's. Cast steel beds, roller bearing axle boxes, mechanical lubricators, self adjusting wedges and pivot points were all intended to increase their availability. They were designed to operate over 60lb rails but most of their working days were spent on main lines where a heavier axle load would have been an advantage. Most GMA's were converted to GMAM by increasing their coal and water capacities. This only increased the axle load marginally and although a very useful locomotive they would probably have had longerGL Garratt, SAR's largest and most powerful steam locomotives. The GMAM's have always had an auxiliary lives had they been designed as a modern version of the water tank permanently attached similar to the GM Garratts.

The GMAM's have seen service on many of the steeply graded and curved lines of SAR as well as on many main lines. However they started being withdrawn in the late 1970's as lines were rebuilt with heavier track and a need for this type of light axle load locomotive diminished.

Today only three remain in service on SAR, retained for special duties although several are in industrial service. These are at Randfontein Estates Gold Mine which operates 12 of these locomotives hauling 1000 ton block loads of gold ore on its extensive system near Johannesburg and Tweefontein Collier near Witbank which has eight.