  We use high-resolution angle-resolved photoemission spectroscopy to map the
three-dimensional momentum dependence of the superconducting gap in FeSe. We
find that on both the hole and electron Fermi surfaces, the magnitude of the
gap follows the distribution of $d_{yz}$ orbital weight. Furthermore, we
theoretically determine the momentum dependence of the superconducting gap by
solving the linearized gap equation using a tight binding model which
quantitatively describes both the experimental band dispersions and orbital
characters. By considering a Fermi surface only including one electron pocket,
as observed spectroscopically, we obtain excellent agreement with the
experimental gap structure. Our finding of a scaling between the
superconducting gap and the $d_{yz}$ orbital weight supports the interpretation
of superconductivity mediated by spin-fluctuations in FeSe.
