... showed them how to measure the distance of conspicuous objects out of doors by means of a measured baseline. In this way Gray, when a boy, measured the distance of Nelson's monument on the Calton Hill, the lighthouse on the island of Inchkeith, the Martello tower at Leith Harbour, North Berwick Law, and other objects that can be seen from Burntisland. The practical nature of the teaching intensely interested the boys. In later life Gray always tried to make sure the problems he set the students as practical and as humanly interesting as possible.

Gray ever put duty before personal ambition, and did not seem to mind.

Gray took a leading part in the testing of dynamos ... and in the testing of accumulators and of electric lamps.

... that not only had he written out the solutions of those questions which had been answered incorrectly, but he had also written out complete solutions of all those which he had not attempted.

While in Wales, he championed the cause of the higher education of women, and took a leading part in the foundation of the County School for Girls in Bangor. At this time he was also an enthusiastic mountaineer, and made weekly excursions with some of his colleagues into the Welsh hills. He was a strong swimmer, and rarely missed his morning bathe in the Menai Straits.

Gray's principal achievements were as an indefatigable teacher, including extension teaching, and university administrator. Where Kelvin might easily have fallen foul of modern teaching quality assessments, Gray's attention to detail, his advice and guidance to students, kindness, and pastoral care would have seen him through with flying colours. As a result the number of students taking the various natural philosophy classes in the university increased enormously, though few of them were inspired to greatness as many were by Kelvin's exciting but, for the ordinary student, execrable lectures. Gray reorganized the obsolete department he inherited at Glasgow, and founded the Natural Philosophy Institute in 1906, the largest building devoted to physics in Britain; by 1923 more than 600 students a year took laboratory courses there.

... did much to make experimental electricity an exact science ...

His organizational exertions on numerous committees during the First World War, the death of a son in 1915, and the demands of vastly increased student numbers after the war undermined Gray's already uncertain health and he resigned his chair in 1923. He died on 10 October 1925 at 15 Victoria Circus, Glasgow. He was survived by his wife [three sons and four daughters]. It was a source of satisfaction to him that James Gordon Gray, his second son, had renounced the engineering profession for physics and been appointed professor of applied physics at Glasgow.