A comprehensive understanding of gas flow in long hollow-core photonic crystal fibers (HC-PCFs) is critical for evaluating their sensing performance for low-concentration gases, especially in terms of response time. The aim of this paper is to numerically and experimentally investigate the pressure-driven gas flow dynamics in a relatively long HC-PCF-based gas sensor. The gas flow in the core of a 1.1 m long HC-PCF was numerically modeled to examine the gas sensing response time in terms of the time for the gas to fill the core (gas filling time). The model was validated against the experimental results of continuous-wave modulated photothermal spectroscopy. The model was then used to analyze the effects of gas inlet pressure, core diameter, fiber length, and gas type on the gas flow field and gas filling time. The results revealed that a lower gas filling time was achieved as the pressure difference between the inlet and outlet increased, the core diameter increased, and/or the core length decreased. The developed numerical model provides valuable information such as cross-sectional velocity profiles and gas flow rates that cannot be readily obtained from simpler analytical models.