The name Gabriel is actually an odd one. It's a relatively ubiquitous name, coming in at 24 on the most popular boys' name list in the US in 2008. And yet, although the name is important to Judaism, Christianity, and Islam, it is only recently that it has become popular.
In the Old Testament, Gabriel the angel is sent by God to fight Jacob, but in fact to help Jacob find his calling. In the New Testament, Gabriel the angel announces to Mary the birth of her son, Jesus. And according to Islam, the angel Gabriel dictates the Qur'an to Muhammad.
Still, even with its history, the name has only become a Top 100 name in the US in the 1970s.
One thing that comes to mind is that although Gabriel has been gaining popularity, it is also passionately disliked by many. The name calls attention to itself, and not all parents feel comfortable giving their kids a name that will define them as Gabriel may do.
2009 Update:
The name has continued its rise, ending 2009 in the 22nd spot.
2010 Update:
Another small rise, ending at 21.
