Well, inherently at room temperature it's a colourless gas, but to explain this further with a comparison to another substance.
Suppose we have HF which is a liquid and HCL which is a gas. This is because HF has stronger intermolecular forces of attraction than HCl does. As such, since the molecules are more strongly attracted, they have to have a greater average kinetic energy in order for them to escape as a gas. This means the molecules will find it harder to move around because a solid has molecules closely knit together like close friends who can never break apart, a liquid has molecules which move more without much restraint and a gas has molecules which basically fly everywhere like their life depends on it (sorry for the bad analogy lol). Furthermore, HF has hydrogen bonding between the H on one molecule and the F on another molecule. Due to Cl's lower electronegativity and larger size, it cannot undergo hydrogen bonding, so the dipole interactions between the HCl molecules are not strong enough to keep that molecule as a liquid at room temperature, hence it is a gas.
