In cases where classical physics provides a close approximation to the true spectrum, the Gaunt factor is close to 1.
When quantum physics becomes important, it becomes bigger or smaller than 1.
[1] The Gaunt factor was named after the physicist John Arthur Gaunt, based on his work on the quantum mechanics of continuous absorption.
[2] Gaunt used a 'g' function in his 1930 work, which Chandrasekhar named the 'Gaunt factor' in 1939.
[3] It is sometimes named the Kramers-Gaunt factor as Gaunt incorporated the work of Hendrik Anthony Kramers.