I am also a descendent of Francis Gray and Elizabeth Collin Thome. Their son Johnathan is my great, great grandfather.
I believe Francis Gray was born in 1834, in Cornwall (I think). He married Elizabeth Collin Thome on 21st July 1864 in Kandy, Sri Lanka.
They had 9 children: Frank, Johnathan, Wally, Charles, William, Elizabeth, Mary, Rose and Rebecca.
Elizabeth Collin Thome parents were William Thome and Hannah Forster. Hannah Forster's father was Richard Forster, an officer in the British 83rd Foot Regiment in Sri Lanka.
Francis Gray died on 10th March 1882. Elizabeth died on 7th September 1883.
I hope this information is useful for your cause.
I too am looking for more information on Francis Gray. Have been trying to find his parents and place of birth for some time now, but have been unsuccessful.
I would appreciate receiving any information you may have that can be of use to me, in particular for Francis Gray. However I would also be interested in receiving some information of your family tree from Mary Gray through to yourself and family.
My father and I have been constructing a family tree book of Grays, but as yet do not have any information on Mary Gray at all.
Please feel free to contact me direct on firstname.lastname@example.org