In this paper, we analyze the performance of diffusion-based gas distribution in antiresonant hollow-core fiber-based gas absorption cells. Performed theoretical analysis was based on Fick’s second law using the OpenFOAM® software and finite volume method (FVM), followed by an experimental verification of the obtained simulations. The diffusion time was tested for a 1.25 m long fiber, with laser-micromachined microchannels. Full analysis of the correlation between the microchannel count, position, and separation on the rate at which the fiber-based gas cell was filled with the target gas was presented. Experimental results showed that with the proper microchannel configuration, the purely-diffusion-based gas exchange time in the 1.25 m fiber could be reduced from 6 h, down to 330 s. Obtained results correlated with the simulations, giving perspective for the development and implementation of novel miniaturized passively filled gas absorption cells for compact laser spectrometers.