Question

Dry hydrogen chloride gas does not effect a dry strip of blue litmus paper but it turns red in the presence of a drop of water ?

Answer 1

Any acid is said to be an acid if  there is a proton acceptor to accept the proton donated by the acid. Since water accepts a proton to form a hydronium ion, the acid is ionized.
A dry strip of litmus and dry HCl in gaseous form do not contain water so HCl does not ionize, but in presence of a drop of water, HCl is ionized and releases a proton which turns blue litmus red.

Answer 2

Yes because dry HCl cannot give H⁺ ions which is the acidic ion and responsible for the change of litmus paper.But if a drop of water is given to the litmus paper then it will give red coloration with dry HCl because now it can dissociate into H⁺ and OH⁻ ions,which give red coloration to the litmus.