A gas-burning appliance includes a combustor and a flow guide device engaged with the combustor. The combustor has a gas outlet. The flow guide device includes a separator and two stop plates. The separator has an opening. Each of the stop plates is located at the opening, and a top edge thereof is higher than a top surface of the separator. The combustor is located below the separator with the gas outlet corresponding to a space between the stop plates. The flow guide device has at least one first air inlet, which is located below the separator, and communicates with the space between the stop plates. A gas fireplace includes a firebox, a translucent shield, and the gas-burning appliance. A separator divides the firebox into an air chamber, which receives the combustor, and a combustion chamber. Whereby, the visibility of flame and the combustion efficiency could be improved.