The importance of Global Navigation Satellite System (GNSS)-based earthquake early warning for modeling large earthquakes has been studied extensively over the past decade, and several such systems are currently under development. In the Pacific Northwest, we developed the Geodetic First Approximation of Size and Timing (G-FAST) GNSS-based earthquake early warning module for eventual inclusion in the United States west-coast-wide Shake Alert system. We also created a test system that allows us to replay past or synthetic earthquakes to help identify problems with both the network architecture and the algorithms. Here, we report on the performance of G-FAST during three large megathrust earthquakes in Chile: the 2010 M-w 8.8 Maule, the 2014 M-w 8.2 Iquique, and the 2015 M-w 8.3 Illapel. Magnitude estimates based on peak ground displacement would be available between 40 and 60 s after the origin time for the three earthquakes, with a magnitude bias less than 0.3 magnitude units. Centroid moment tensor and finite-fault-slip estimates show good agreement with prior results and are available between 60 and 90 s after the origin time. Furthermore, we test the impact of epicentral location errors, latency, and data dropouts on the robustness and timing of alerts and show no significant variability in the results. Finally, we discuss implications for earthquake and tsunami early warning in Chile.