It depends on the type of gas heater you have. There are generally two main types of gas heaters: vented and unvented (vent-free) gas heaters.
Vented Gas Heater: A vented gas heater requires electricity to operate the blower fan or other electrical components that aid in the combustion process or the distribution of heated air. Without electricity, the fan won't work, but the heater's main burner should still produce heat. However, the heat output will be significantly reduced compared to when the fan is functioning, and the distribution of warm air will be limited to natural convection. Vented gas heaters are designed to be used with proper ventilation to remove combustion byproducts.
Unvented (Vent-Free) Gas Heater: Unvented gas heaters are designed to operate without a venting system and are more dependent on electricity for their operation. They use oxygen from the room for combustion and release heated air directly into the living space. These heaters typically have oxygen depletion sensors (ODS) and a thermocouple safety feature that requires electricity to function properly. Without electricity, the safety features may prevent the heater from operating altogether. It's crucial to note that using unvented gas heaters without proper ventilation can lead to a buildup of indoor pollutants, such as carbon monoxide, which can be dangerous.
In both cases, it's essential to refer to the manufacturer's specifications and safety guidelines. Never use a gas heater without proper ventilation or if it is designed to rely heavily on electrical safety features that may not function during a power outage. If you're concerned about using a gas heater during a power outage, consider having alternative heating options available or ensuring that your gas heater has a battery backup system for critical electrical components.