Dear Falk,
Thankyou for the info. Today I found a Mary Gebhardt on the 1870 Mo. Census which listed her age as 19. I think this might be my Mary. The fathers name is listed as John and Mother also Mary, with siblings, William, Lizzie, Louise, Emelia and Adam. It also says she was born in Missouri which could be mistake. Says father is from Hesse Darmstedt and mother Prussian.
My Mary Gebhardt married a MeyerPeter with name later changed to Meyer/Meyers. On the 1880 census I find a William Gebhardt with an Elizabeth MeyerPeter as mother-in-law. (In same city). I had also noted a Louisa Gephardt/spelling living with an August Meiers as a domestic in 1880. My Mary's husband later was known as August Meyers. So I thought perhaps she may have had 2 names, but if I found the correct census she had a sister names Louisa. As you can see I am speculating. 
As far as speculating, I also have a circa 1900 photo taken in St. Louis of a Lizzie Brennan. Perhaps it is Lizzie Gebhardt-Brennan.
Just could be that Mary and and her brother married siblings. I don't think that is uncommon in the 19th century.
May I ask you to look if there is a John Gebhardt? in 1856 he would be about age 30. His wife would be Mary also age abt. 26. Sinse I believe my Mary was a small child when she immigrated the year 1856 may be incorrect, but probobly close.
This newly found info makes me think my Mary's fathers name might have been John/Johan Gebhardt. Sinse they did ultimately live in Missouri, I don't think they would have came through Canada. More likely New Orleans, though I can't be sure.
Warmest Regards,
Barbara