In our past, and yours, I assume, Gabriel Gray was a watchmaker from Queens, New York. At some point, that I'm only starting to figure out when, he was pushed to the breaking point and snapped. He killed one man and took his ability - see, Gabriel's ability is to see how things work. This is extended to see how we work. Or, rather, how our abilities work. By examining the brain (after removing it from the skull), he was able to take the ability for his own. After he got the first, it became an obsession, and he kept killing our kind to absorb more and more abilities.
So, in a way, Sylar is a potential future for Gabriel. One that we are all hoping like hell to avoid.