Interestingly, Gabriel has the exact amount of Health and Sanity he has on Arkham Horror 3rd Edition, but his ability is completely different (in the board game he was basically a reskin of Leo De Luca)

He is he good however? Extremely: one curse to draw an extra card is incredibly good, it makes your deck that much faster with a fairly manageable drawback. It can disrupt bigger player groups, so you probably should tell your team ahead of time you are planning to use him, but he also has the advantage of being a reliable source of curses if you want to get for a "Curse Seeker" build, which originally wasn't quite worthy to do because Seekers just didn't have a constant, renewable source of cruse tokens: Gabriel alone will be the backbone of decks using Gaze of Ouraxsh, The Stygian Eye, Fey and Prismatic Spectacles.

AND on top of that, it increases your intelect by 1!

Even if you are not doing curses, Gabriel is a very solid ally for the draw and the stat boost, I can see people upgrading into him in place of Jeremiah Kirby (if they need more consistent draw) or Dr. Milan Christopher (if they do not need the extra resources)