He then joined the staff of the Department of Materia Medica in Edinburgh under physician and pharmacologist Sir Thomas Richard Fraser who with chemist, Alexander Crum Brown FRS, made systematic attempts to study the structure-action relationships of drugs.

and conducted research for the Ministry of Munitions into the action of irritant gases on bronchi, and on poisoning by salvarsan used for treating syphilis.

[7] Gunn's proposal was supported for refurbishment for teaching and research facilities, and to expand the departmental library.

In 1931 Gunn initiated the British Pharmacological Society with a letter signed with Sir Henry Dale and Dr. W. E. Dixon.

Gunn's main line of research was in adrenaline starting as a student of Sharpey-Shafer who was appointed chair of physiology in 1899 in Edinburgh.

Gunn investigated the action of the amines which formed a series beginning with phenylethylamine and ending with adrenaline.

[11] Gunn moved from pharmacology in 1937 to become the first director of the Nuffield Institute for Medical Research in the University of Oxford.