Conventional gas diffusion measurements in coarse-textured and aggregated porous media are severely limited due to hydrostatically induced variations in water content and air-filled porosity. Motivated by the need to measure gas diffusion in coarse-textured plant growth media designed for use in microgravity (e.g., aboard the International Space Station), our objectives were (i) to develop and test an automated diffusion measurement system on earth with water content adjustment capability and that minimizes hydrostatic effects, and (ii) to model characteristics of gas diffusion in partially saturated aggregated porous media. The horizontally oriented O2 diffusion cell design for reducing the gravitational effect was based on a thin profile rectangular cell. Continuous measurement of O2 in sealed dual-chamber diffusion cells provided concentration data for fitting diffusion coefficients where water content was controlled volumetrically using a porous membrane with an imposed pressure for incremental addition or removal of water. Gas diffusion was modeled as a function of air-filled porosity in millimeter-sized aggregated particles exhibiting a substantial difference between internal and external aggregate pore sizes. For this case, the internal aggregate porosity contribution to diffusion compared with external aggregate pore space was minor as described by a dual-porosity diffusion model. The measurement approach described can be used in other coarse-textured and structured porous media.