The gas involved in the reaction between sodium trioxocarbonate IV (also known as sodium carbonate, Na₂CO₃) and dilute hydrochloric acid (HCl) is carbon dioxide (CO₂).

Here's the balanced chemical equation for the reaction:

Na₂CO₃ (aq) + 2 HCl (aq) → 2 NaCl (aq) + H₂O (l) + CO₂ (g)

The reaction produces carbon dioxide gas, which you can observe as bubbles forming in the solution.