The South African arms industry was supplied shells and technology to produce the advanced 155mm during the Angolan Wars, by Space Research Corporation (SRC) of Canada/USA.
This landed Gerald Bull in prison for illegal export.(The PBS Frontline episode on same, or less-so the “Doomsday Gun” has some pretty general information on the subject. See: http://www.pbs.org/wgbh/pages/frontline/programs/transcripts/911.html 
They got technology to produce Bull’s more up-to-date gun too (and produced two types: one a 155mm, the other a 210mm). It wasn’t necessary to have to use Bull’s 155mm base-bleed shells in his GC-45 as they could also be used to great effect in existing 155mms. It was called the G5 in South Africa (or GC-45, GN-H-45) it was a mobile artillery weapon with better ballistics and hence longer range – most importantly better accuracy than the conventional 155mm guns up to that time – early to mid-80s. The 210mm was self-propelled. It is true that Iraq may have gotten 155mms from SA, but they may have more likely gotten the shells only (still better in the “older” guns) – but what good were they to the Iraqis in the final math of the first Gulf War?
Gerald Bull designed this and other weapons and for a number of complicated reasons was assassinated in March, 1990 in Brussels.
The South African arms industry didn’t design it – this wasn’t suggested per se – just wanted to clarify where the design really came from. – Fitz
JWR adds: This whole sordid saga is a fascinating bit of recent history. “Base bleed” technology is fascinating. Most rifle shooters know that boat tail bullets have less drag, because with less rear surface area, there is less suction. Artillery shells suffer from the same drag, but on a grand scale. Gerald Bull’s solution: a small, slow-burning, rocket-like charge that counteracts the suction, and hence greatly increases the range of artillery shells. A brilliant concept.
Coincidentally, this story is also a tie-in to another recent SurvivalBlog topic: border straddling. The SRC Corporation’s headquarters were on an 8,000 acre parcel that straddled the U.S. Canadian border. I’m sure that made “import-export” issues a breeze. 😉