History of the class

An excellent and in depth article on the GMA(M) can be found on WikiPedia. I feel no urge to improve on that. To freeze it I retrieved it from WikiPedia as a PDF-document.

South African Railways - SAR "Beyer Garratt" type GMA/M Class 4-8-2+2-8-4 steam locomotive Nr. 4054 (Henschel Locomotive Works, Kassel 28683 / 1953). Official Henschel (Kassel) works photo, 1953.
Source: https://www.flickr.com/photos/124446949@N06/45050014085/

In short

The class GMA was built between 1954 and 1958 and consisted of no less than 120 members. It was to be the most numerous Garratt class ever and to the best my knowledge the only class ever to exceed 100. It was built in several batches by Beyer Peacock, North British Locomotive Company and Henschel.

The engine could be configured as GMA with a slightly lower driver axle load or as GMAM (the extra M for Mainline) with slightly higher axle load. This was done by adding plates, limiting water and coal supplies. This could be done so easily that is nigh impossible to know which engine was a GMA and which a GMA/M at any given point in time.

They proved to be hard working and reliable engines, though a bit short of water supplies. They were soon semi-permanently coupled to auxiliary water tenders (X-17 and X-20)..

Comparison

I have made a comparison between the GMA/M and various other classes. Admittedly the chosen other classes are a little subjective. I chose the EAR 59 as it is the largest Garratt ever built. The AD60 comes into play to make a comparison with a well known standard gauge locomotive. The Big Boy because it is generally considered as the epitome of all things steam and I included the strongest and latest Dutch freight class.