He served as keeper at the department of entomology in the British Museum (Natural History) for thirteen years after Charles Owen Waterhouse.
His father, Michael Gahan was the Master of Erasmus Smith's School in Tipperary.
He was educated first at Queens College Galway, where he achieved distinction, and then at the Royal School of Mines in Kensington.
An expert on beetles, especially Cerambycidae, he wrote the 1906 volume of The Fauna of British India, Including Ceylon and Burma on that group.
Gahan served as honorary Secretary of the Entomological Society of London in 1899-1900 and was president from 1917–1918.