I see you have added that service record I found to your tree, it says that his parents are listed in the next of kin section on enlistment as parents Michael and Sarah, then after he married it would be his wife which is a good match for your man.
However, I know very little about army uniforms and Wexflyer says the man wearing uniform on the photo was an officer so I'm not at all certain this could be 'your' Michael McGrory jnr in the photo as the service record says he was a Bombadier (or Lance Bombadier) so maybe attaching the photo is a bit of a long shot unless you have other info to confirm this?
Is that 'letter' actually a letter or is it a business card?
It looks like a business card which has M.A. McGrory's job title, employer and business address and also his Pvte (home/private) address. of Ingleton, Aigburth Hall Road, Liverpool
The Garston 1917 is not a date but a home telephone number. I haven't looked for other years but there's definitely one in the British Phone Books, 1880-1984 on Ancestry (which you seem to have access to) matching this name, address and tel no in 1946
FindMyPast have images for the marriages at parish church at Buckland in the record set kent-marriages-and-banns and this marriage record will be in there and will give you their fathers' names and occupations.
a free search of the 1921 census shows a Michael Aloysius McGrory (born 1893, Armagh) and a Gladys Rosa McGrory (born 1895, Chichester, Sussex) living in Liverpool . That would match with the possible service record I posted. Now though we know your Michael jnr was born in Gateshead and not Armagh, he did grow up in Armagh and is on the 1901 census there
https://www.census.nationalarchives.ie/pages/1901/Armagh/Clady/Cladybeg/1016605/If you don't have access to FindMyPast then maybe your library does or you could sign up for a free trial and get the images for both the marriage and the 1921 register
The couple are listed on the 1939 register in Chichester and give birth dates (though the original date for Michael was later altered by the NHS)
I'm nosey and downloaded the birth register entry for the Michael McGrory who was born in Gateshead as additional verification - I've attached it for you.
Boo