I went to familysearch.org to find the names of the children as they weren't listed in the post.
William Francis baptism 1878
Amelia Caroline, baptism 16 June 1878
Agnes, baptism 12 Oct 1879
Bedwelty Monmouth England
Can you confirm these are the children of William and Amelia. This will at least place the family for 1878 and 1879.
Also
1881 Census
William Gosen, age 36
b. 1845 Canada, lodger, married, general labourer
Address - Graig Berthllwyd Cottage
Llanfabon, Glamorganshire, Wales
Head of househould is Edwin Boulton
I would suggest that this is the William Gosen married to Amelia and now we have his year of birth 1845.
All of the above can be very helpful in trying to find where he was born and married.
I have gone back through most of the census info availalbe through automated genealogy and library and archives Canada. My best guess and I am say GUESS would be to concentrate on Ontario, probably the Frontenac and Waterloo areas. I would search for Gosen/Gossen and alternative spellings for anyone with the names William, Francis, Agnes, Caroline as many people used a naming pattern and that may be the only way to narrow down a search.
I would also find the source for the info on family search and contact to see if you can find the original baptism registrations as that could give more information.
I would get the marriage registration details (not the certificate) for Amelia and Richard Saunders. The registration will more than likely have more details than the marriage certificate which usually only has the date and place. Registration details can give place of birth, parents, etc.
There is a death on Family Search for a William Gosen 1910 age 65 (age out by 10 years) Pittsburgh, Frontenac, Ontario
Hope this helps some.