At room temperature, pure ammonia is a gas. BUt since pure ammonia has boiling point reasonably close to room temperature, it doesn't take much compression to liquefy it. At room temperature, the condensation pressure looks to be ~7.5 bar, so say approximately 7.5 atmospheres. As you can imagine, pressure of ammonia inside a standard cylinder is quite a bit higher. So although we call it a "gas cylinder", the ammonia gas is actually liquefied by the pressure. Of course, it's usually dispensed at a much lower pressure, so it may come out of the tank as a gas.
You might google "phase diagram of ammonia" and you will see what I mean.
It doesn't help that in common parlance, ammonia dissolved in water is often called "liquid ammonia" or just "ammonia".