Greg,
The GMAM class was a 1950's design (and bloody ugly with it!) whereas the GL's were built in 1928/30. I had a look at the GL preserved in Manchester last year while I was on business nearby and was surprised how small it was. It doesn't look too much larger than a K36 or K37 (weight 472 864lbs).
A GL was a purely freight loco and I've seen them described as the "ultimate cart horse". The original deign brief was to equal 2 14 class or 3 electrics on the Natal mainline which, as you are probably aware, is not exactly flat.
I also got to see the GMAM's at REGM, though in my case this was in 1988.
G
Picture is of GL, 2351 on a test run after restoration at Bloemfontein