  We propose a method for computing generating functions of genus-zero
invariants of a gauged linear sigma model $(V, G, \theta, w)$. We show that
certain derivatives of $I$-functions of quasimap invariants of $[V //_\theta
G]$ produce $I$-functions (appropriately defined) of the GLSM. When $G$ is an
algebraic torus we obtain an explicit formula for an $I$-function, and check
that it agrees with previously computed $I$-functions in known special cases.
Our approach is based on a new construction of GLSM invariants which applies
whenever the evaluation maps from the moduli space are proper, and includes
insertions from light marked points.
