Lysozyme crystallisation was first-time performed in a microfluidic device in the presence of different gases: helium, nitrogen, oxygen, and carbon dioxide microbubbles. It was found that protein adsorbed on the gas–liquid interface stabilised the gas bubbles in the aqueous solution, and bubble stability increased with the protein concentration in the solution. The heterogeneous nucleation of protein on the gas–liquid interface was preferred than on the capillary glass wall, limiting the fouling inside the capillary. The crystals formed with curved surfaces, and the crystals floated in the solution with gas bubbles. The population density of lysozyme crystals increased with an increase in the solubility of four types of gases. Three stages of the protein crystallisation on the gas–liquid, gas–solid and liquid–solid interfaces were discussed.