The gas produced when hydrogen carbonate (also known as bicarbonate) reacts with acetic acid is carbon dioxide (CO₂).

Here's the chemical reaction:

NaHCO₃ (sodium bicarbonate) + CH₃COOH (acetic acid) → CH₃COONa (sodium acetate) + H₂O (water) + CO₂ (carbon dioxide)

This reaction is commonly seen when baking soda (sodium bicarbonate) is added to vinegar (acetic acid). The fizzing and bubbling that occurs is the release of carbon dioxide gas.