We may be looking at an alias here. He appears out of nowhere with an address in Croydon. Mrs Gardiner may well be his mother. He could be Benjamin Gardiner, who is the only one of the children whose age fits. This is the most likely answer, but it would be difficult to prove.
1884 Jun 14. Born (Brighton from service record of O'Shea) There is only 1 George O'Shea born in England between 1880 and 1890. He was born Poplar in 1884
1891 census at ?
1901 census. Family living in Croydon
1902 Apr 21. Enlists in Dragoon Guards (this is the correct service record). He gives there Father George, 1 Part St, Brighton, older brother Fred, younger brother Harry
1903 Feb 28. Receiving Stolen Goods sentenced 56 days in prison
1905 Apr 21 Transferred to Reserve
1911 census there is a Mrs Gardiner at the MIC Address (her maiden name was Norman and she married in 1877). Benjamin is not there
1914 Apr 20 Discharged at end of service from Reserves. He puts his mother as Fanny Sarah O'Shea, living in South Africa.
1915 Sep 15. Landed in France
1917 Dec 13 Commissioned 2nd Lt.
1919 Feb 13. Resigns his commission and gazetted out as 2nd Lt. His Protection Cert gives his work as "Music Hall Artiste"
1920 Nov 8. Joined ADRIC with service no 977. Posted to I Coy. Next of Kin, Mrs Gardner, 35 Chelsam Rd, Croydon (as on MIC)
1921 Dec 16. On the raid on Ceannt house where items were reported stolen
1921 Mar 20 Promoted Section Leader
1921 May 21 Posted to Depot
1922 May 22 Admitted Stevens Hospital
Virtually all the rest of his time with ADRIC is on Medical Leave
1921 Sep 24. Permitted to resign on compassionate grounds