Do gases have definite or indefinite shapes?
Have you never seen a balloon folder?
You know those guys who take the long balloons, blow them up, and make swords, and party hats, and dinosaurs, and all sorts of complicated shapes. The point I am trying to make is that the gas-filled balloon has an indefinite shape, and it is modified by the shape that the balloon-folder creates.
There are many gas laws derived from observation and experiment over the centuries, and these have establlished that a gas (or mixture of gases) will fill the volume of any container uniformly. Gases thus have indefinite shapes, dependent on the container.