Pwn used to be pronounced pween because it originated as an april fools joke in PC Gamer magazine in the mid 90's. When the april edition of the mag (mind this was when it was a hugely popular product) came out and every article and review was using the word everyone was like 'wtf' until they finally said it was an april fools joke, they wanted to invent a new word for kicking ass, the mag even included how to pronounce the word. But as you can see the use of pwn quickly spread.
Today the new kids say pown because they all think it is a derivative of own. The simple fact that it is pronounced like this in major media when it is rarely said doesn't help.
It's similar to how the folks use leet speech and think it is cool because it makes the words look all funny and cool. When it was originated so that BBS users could still search for porn after that word was banned lol. Thus the first leet speech word was pr0n.
On topic with Gib, everyone I have ever played quake with has said it with a hard g, so that is how I pronounce it. I'd be willing to bet that while Carmack says it is a j sound, others within iD say it differently. So I chalk it up to dialect differences.