After a bit more digging your Great Grandfather enlisted into the 6th Bn Manchester Regt on 26.06.1913 and was given the number 1759. He was discharged on 15.05.1915 as no longer being fit for war service. He was awarded the 1914-15 Star, British War Medal and Victory Medal for his service in Egypt between 10.09.1914 and 15.04.1915.
He re- enlisted into the Royal Field Artillery(Territorial) and was first given the number 2720, he was renumbered in 1917 and given the number 706498.
On 30.03.1921 he re-enlisted into the Royal Artillery for 1 year and was given the number 738856. He again re-enlisted on 28.11.1939 and was finaly discharged on 25.01.1944.
He has discharge papers, from the Manchesters, on Ancestry and his 1921 enlistment is recorded in the RA Attestations book on Find My Past. They both have free access today. There may be further records at the MoD
https://www.gov.uk/government/collections/requests-for-personal-data-and-service-recordsCharlie