  For toric Calabi-Yau threefolds, open Gromov-Witten invariants associated to
Riemann surfaces with one boundary component can be written as the product of a
disk factor and a closed invariant. Using the Brini-Cavalieri-Ross formalism,
these disk factors can often be expressed in terms of gamma classes. When the
Lagrangian boundary cycle is preserved by the torus action and can be locally
described as the fixed locus of an anti-holomorphic involution, we prove a
formula that expresses the disk factor in terms of a gamma class and
combinatorial data about the image of the Lagrangian cycle in the moment
polytope. We verify that this formula encodes the expected invariants obtained
from localization by comparing with several examples. We then examine a novel
application of this formula to disk enumeration on the quintic 3-fold. Finally,
motivated by large $N$ duality, we show that this formula also unexpectedly
applies to Lagrangian cycles on $\mathcal{O}_{\mathbb{P}^1}(-1,-1)$ constructed
from torus knots.
