Alan,

Yes, language is interesting. I still need to go back into the book and see what Gordon specifically was talking about. In the meantime allow me to offer my own, and I'll fully admit this my own distinction based on an 'artistic license' I give to those 1,000% and more creative people who spend their hours and days creating full dress Atlantic Salmon masterpieces.

'Dressing' is the act of: a)blending multi colors of numerous natural, and perhaps some synthetic materials, b)applying/attaching those materials to a hook with thread using hand and finger dexterity to create a fly that is beautiful to look at with the human eye, functions as a fly insofar as fish catching if desired, is basically a work of art. It may also but not necessarily involve the actual manufacture of a specific design of hook.

'Tying' is the act of preparing flies most specifically for fishing. This means there may be little more than a tuft of fur on a hook or it may mean a fly that is the full 'enchalada'. In other words: 'All Dressing is Tying, but not all Tying is Dressing'.
I sincerely hope I haven't offended any of the great contemporary 'fly dressing masters' or ones of years past. Also, I wonder if the above distinction makes sense to anyone?
I still need to find what Gordon wrote and hope to report it when I do.

Allan