Finding that Aberdeen afforded him no sufficient field for practice in competition with his elder brother, Gregory went in 1754 to London, where he had already friends such as Wilkes and Charles Townshend, whom he had met at Leyden, and where he speedily made other friends, of whom may be mentioned George, Lord Lyttelton, Edward and Lady Mary Wortley Montagu. He was at once elected into the Royal Society, and would no doubt have gained fashionable support; but his elder brother dying in 1755, he was recalled to Aberdeen to fill the Professorship of Medicine. Here he continued to practise and to lecture till 1764, publishing in the latter year “A Comparative View of the State and Faculties of Man with those of the Animal World.” He then removed to Edinburgh with a view to securing a professorship there. This fell to his lot in 1766, on the death of Rutherford. In the same year he succeeded Dr. Andrew Whytt as physician to the king in Scotland. He at first lectured on the Practice of Physic solely, but in 1770 he agreed with Cullen that they should lecture in alternate years on the Theory and the Practice, and this arrangement was continued permanently. As a lecturer he was very successful, simple and not in any way oratorical in style. He was especially noted for some lectures on the “Duties and Qualifications of a Physician,” which were afterwards published, and went through several editions. He gave the profits to a poor and deserving student. In 1772 he published “Elements of the Practice of Physic,” a kind of syllabus of lectures; and this completes the list of his medical works. His name was more known after his death as the author of a little book of advice to young girls, “A Father’s Legacy to his Daughters,” which has gone through very many editions. His tone may be judged from the following extract:— “Do not marry a fool; he is the most untractable of all animals; he is led by his passions and caprices, and is incapable of hearing the voice of reason.... But the worst circumstance that attends a fool is his constant jealousy of his wife being thought to govern him. This renders it impossible to lead him; and he is continually doing absurd and disagreeable things, for no other reason but to show he dares do them.... A rake is always a suspicious husband, because he has known only the most worthless of your sex. He likewise entails the worst diseases on his wife and children if he has the misfortune to have any.”