How to estimate HCN content in a volatile mixture?
From a quick search of the literature, it seems that as you correctly thought, gas chromatography mass spectrography is a method that can be used to quantitate small amounts of cyanide in vapour.

While I am certainly no expert on this field or the precise method that you would use, this paper, which describes quantitation of cyanide and other chemicals in tobacco smoke may be of interest: A simple GC–MS technique for the analysis of vapor phase mainstream cigarette smoke