I think it's obvious you assume everyone knows Gabe's personality. I for one only heard his name in news. Never seen his picture, read his interviews, etc.
Considering he created and runs an incredibly successful game development and distribution company who's target market is people under 25 years old it's pretty reasonable to asssume that he has half a clue about his target market.
comScore also released an announcement a while ago saying that gamers actually represent all markets segments, showing the potential of in-game advertising (which has been growing both in revenue and number of advertising in the past years).
So Gabe's target market should be a very broad one not only gamers "under 25 years old", specially since his main product is a platform (Steam).