There are two types of heaters: indoor and outdoor heaters. Indoor gas heaters are very safe when properly maintained and used as instructed. The same is true for LPG-LPG gas heaters are safe too.
Never use an outdoor gas heater indoors, as they are not designed for safe inside use. Keep your gas heater safe by having it serviced by a qualified gas fitter at least every two years.
But honestly, gas heaters emit carbon dioxide into the room and increase the level of indoor pollutants which can potentially harm human health including high incidents of respiratory symptoms.