I've got the same problem, Mozilla, chrome and MSN give me the same screen. I can access Gumtree UK, France, Malasia etc but not SA. I've had numerous discussions with the admin there who initially tried to say it was on my end.
They modified there site so you can only access it from inside South Africa. Pretty stupid if you ask me as all of us that work out the country also sell and buy in country. I'm in Nigeria, I've got less than no chance of getting on with my IP address.
Normal South African narrow mindedness. When you travel, you get used to it, but it's a bitter pill to swallow.