It was all conjecture. Until I read the damning last line. It's clear to me: Gabe is it.

Gabe was in the office during the Scranton Strangler police chase (assuming that was the real strangler that got caught).

Gabe goes on to work at Hoolie and then later at The Pied Piper compression start-up. As weird as he is, he never displays any violent or murderous behaviors. I think we can dismiss him as a suspect.

Gabe doesn’t have the wrist strength the strangle anyone. Also he’s too fat, nobody will like him if he’s too fat.