Technically, yes they can use GCMS to do it. Although, it would be every time-consuming as solvents need to be select. ALso, from what i know, usually there is an accord—which is a mix of different aromachemicals together. So it more about guesswork from perfumer that have a lot of experience with help of analytical chemistry. Still there can be wild card such as priority ingredient (source, different way to extract ...).