We propose a method for computing generating functions of genus-zero invariants of a gauged linear sigma model (V, G, θ, w). We show that certain derivatives of I-functions of quasimap invariants of [V θ 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. CONTENTS 1. Introduction 1 2. GLSM setup and the state space 6 3. Compact type GLSM invariants 10 4. Genus zero, two marked points 21 5. Adding light points 23 6. Generating functions 27 7. Examples and comparisons 36 Appendix A. Proof of Theorem 5.6 43 References 48 * (j 1 c * β 1 ) ∪ • • • ∪ ev c n * (j n c * β n ) in Proposition 3.15 is equal to ev *