Erm...that sounds quite dodgy to me. What's the actual title of the GCSE? You mean they just randomly sat you down infront of a PC and you did it, without warning or anything? Doesn't sound like much of a qualification, no offence. ? Quite strange! But, hey, I guess you should be pleased! Maybe your school's doing a pilot scheme for something they want to introduce at a later date...or maybe, it's not really a GCSE at all, they just told you that so you'd put more effort in on the test!
Anyway, I've never ever heard of anything similar, so I'm not sure it sounds all that respectable. But, hey well done for doing well.