I believe the grave was in Barachois, NB near Maine, USA.
NB joined Nova Scotia and the Province of Canada in Confederation on July 1, 1867. This means Gabriel being possibly born in NB or Maine are one and the same before 1867. I am more inclined to believe that he was born here then in France because it doesn't make since that he would join the British Army if he was brought up in France.
Unfortunately, I have not been able to find the burial info on Thomas Pouchee; however, his d.o.d makes it more likely that he was Gabriel's father. Children did leave home back then at an early age. It would not be unheard of for a 12 year boy to leave and fend for himself.
I know the grave wasn't in Bouctouche as that is where my grandfather was from so I would have remembered that.