Can helicopters fly without gas

Answer 1

Not really.

In the case of a loss of engine power (such as running out of fuel) a helicopter pilot can perform an emergency maneuver called "auto-rotation"

In auto-rotation, the main rotor of the helicopter is disconnected from the engine and allowed to spin freely in the airflow. The inevitable downward movement of the helicopter will cause the main rotor to windmill up to a considerable speed. As the helicopter approaches the ground, the pilot can then use this momentum to slow the descent and hopefully land safely.

This is a very difficult maneuver, and helicopters often fly at such a low altitude that there is not enough time to perform it effectively. Helicopter pilots themselves will often admit that autorotation is little more than a controlled crash. For this reason helicopters are kept to a much stricter maintenance and safety inspection regimen than other types of aircraft and helicopter pilots are trained to keep very close tabs on their engine gauges at all times and land immediately at the first hint of danger.