There are several reactions that will occur. NONE of the gas given off is harmful as it is Hydrogen. BUT, given that you don't always know what it is you are truly working with it is best not to inhale the fumes.
The first reaction is the Zn and HCL producing ZnCl2 & H2or zinc chloride, a solid and hydrogen gas.
The second reaction with rust Fe2O3 producing FeCl3 and H2O, a solid and the water.
The third reaction is between the HCL and the metal itself, this produces FeCl2 and H2, a solid and the hydrogen gas.
Not harmful if you don't consider getting blown up harmful
