<ul data-eligibleForWebStory="true">Constructing robust simulators is crucial for guiding policy in fields like healthcare and logistics.Current methods often struggle with generalization and accuracy, especially when using Large Language Models (LLMs).G-Sim is introduced as a hybrid framework for automating simulator construction.G-Sim integrates LLM-driven structural design with empirical calibration.It utilizes an LLM to propose and refine simulator components guided by domain knowledge.G-Sim grounds the simulator in reality by estimating parameters using calibration techniques.It can leverage likelihood-free and gradient-free methods for parameter estimation and simulation-based inference.G-Sim is capable of handling non-differentiable and stochastic simulators.By combining domain priors with empirical evidence, G-Sim generates reliable and causally-informed simulators.This mitigates data-inefficiency and allows for robust system-level interventions in complex decision-making.