It is not safe to vent a gas dryer indoors; however, an electric dryer can be vented inside a house with some consideration to heat and moisture. Venting an electric dryer inside requires a lint filter.
Gas dryers release toxic and potentially combustible gases that need to be discharged outside. Those who are trying to heat an area with a dryer should note that it increases the overall humidity of the room and even sometimes the house that it is vented into. More importantly, note that the chemicals that are used in the drying process are also vented in to the room.