Collisions of gas molecules with the walls of small pores in nanoporous materials can cause the width of gas absorption lines to become wider. Also, the effective absorption path lengths through the gas become longer due to multi-scattering within the nanoporous materials. GAs in Scattering Media Absorption Spectroscopy (GASMAS) is an effective technique that can differentiate between the absorption from the gas within pores and the absorption from the bulk of scattering materials. A Vertical-Cavity Surface-Emitting Laser (VCSEL) emitting around 760 nm is used to investigate the absorption from molecular oxygen gas trapped within mesoporous alumina Al2O3. The pore size of the alumina samples are also characterized based on Barrett, Joyner and Halenda (BJH) method. In this work, we will present the correlation between the pore sizes of the mesoporous alumina samples the width of the absorption line of molecular oxygen gas under different experimental conditions.