Hi Oursong
I'm intrigued by the surname given it's not one I've come across before. As you have found, there are no entries at all on Scotlands People for this name. The original spelling must have got completely lost in the translation when Samuel Snr went to England! Even when I tried a very open wildcard search - Samuel Gordon M*cd* - any time/place - nothing showed.
There are however a few entries on
www.familysearch.org, which include an Ancestral File on the family (you are probably aware of this), 1881 census data for a Samuel Gordon McD (is this the grandson?) and wife and the birth of this Samuel Gordon McD and his sister in Lincoln. It would appear their mother was called Harriet.
My only advice is in order to go back in Scotland, you need to get back to the origins of the surname.
Good luck!
Monica