Yes - that was a while ago. The initial story that I heard from my parents was that the missing brother was John. Since then I received some information from an Aunt who had done some research, that the name was David Fredrick. This agrees with the census information from 1881, 1891 and 1901.
Her notes show another son, "David" who died as an infant in 1879. I suspect that his name was, in fact, "John" as this would fit with the naming system that the Scots used. And I imagine that it would be unlikely that a baby would be named after a deceased sibling. There should be a record of this death in the Archives of Ontario. The last time I was there, I was not looking for a John Gunn who died before 1881. I may have missed the death notice for David Fredrick as well.