The gas tank has to be vented.
As the gas exits the tank air must be allowed to enter the tank to displace the volume of fuel that has exited the tank. Without a vent the tank will only develop a vacuum inside of it and the gas will no longer flow out.
This is the same principal as holding your finger over a straw to hold a beverage inside of it.
The beverage stays inside the straw until you vent it.
As the gas exits the tank air must be allowed to enter the tank to displace the volume of fuel that has exited the tank. Without a vent the tank will only develop a vacuum inside of it and the gas will no longer flow out.
This is the same principal as holding your finger over a straw to hold a beverage inside of it.
The beverage stays inside the straw until you vent it.