Carter at Ben's urgent urging thinks Gabriel is so crucial in stopping the terrorist attack - the only play CTU has - that he breaks out of CTU along with Ben to find him. This in itself is a good part of the story, with Rebecca and Andy's help. Andy, by the way, is quickly shaping up as an excellent character - he has just what you want in terms of tech savvy and sass in a CTU computer geek, a worthy successor to Chloe.
And there are other strong things going on in the story tonight, including Donovan breaking down his father - at least, to some extent - Nicole breaking away from Aisha and her thug, and Amira killing that poor high school kid, after all. (I think having him wake up, but with amnesia, would have been a more interesting move, but ok.)
But looming behind and over all of this is Gabriel. Carter and Rebecca are risking everything to get to him, so my guess is he's someone really important, played by someone really important.
All we know of him is he's former military. But he's likely much more. So here's my prediction: Gabriel is Tony Almeida, who we know is coming back to 24 in Legacy, "mid-season". Gabriel is Tony Almeida, operating under an assumed name.
We'll see ... and I'm looking forward to seeing next week's episode, and every episode after.
See also 24 Legacy 1.1: Dammit! I Liked It ... 24 Legacy 1.2: Heroes and Villains ... 24 Legacy 1.3: First Big Card Revealed