Need to know what gases produced from the chemical reaction between Hydrochloric Acid and Calcium Carbonate?

Hydrochloric Acid = HCl

Calcium Carbonate = CaCO3

Their reaction

CaCO3 + 2HCl ==> H2CO3 + CaCl2

If you use 20 L of 30% dilute HCl, you'll dissolve about 7 liters of CaCO3.

PS. Don't know the density of CaCO3 , so can't convert it into kg.

HCl + CaCO3 ---> CaCl2 + CO2

CaCl2 is a solid. It's not as soluble as NaCl (table salt) but it will dissolve in water. The products are mostly harmless. I'd be much more concerned about the HCl solution that you're starting out with.

CO2 is the only gas produced by the chemical reaction. CO2 is heavier than air, but is pretty easily displaced, and isn't too much of a problem so long as the CO2 concentration doesn't get too high (which isn't likely). Even assuming the room is totally stagnant though, the amount of CO2 should be relatively small. It would just pool around your ankles. That's assuming there's even that much produced by the reaction and the breathing of you and your co-workers. Not a real concern, I think.

What you might have to worry more about might be the HCl solution. It's not part of the chemical reaction, but some of it may get kicked up into the air as an aerosol while using the stuff. I guess you might consider that a gas. Ventilation should handle that though, and any other safeguards required should be written on the containers and on the safety sheets provided by your employer.

Carbon dioxide is the only one. Although carbon dioxide is heavier than air, there is no particular hazard from it in this context; it isn't toxic, not much is produced, it diffuses rapidly, and the slightest breeze will carry it off.