Unfortunately, you will receive many diverse and somewhat antogonistic theories when you ask the etymology of any word.
I have been told that the name Gleason and Gleeson is derived from the youngest son of Nial of the nine hostages, who was known as Glas. Glas is also the Gaelic word for "green" or "blue green." As the youngest, Glas was sometimes given the Irish diminutive "in" rendering the name "Glasin" thus giving rise to the facile "Little Green Man" theory of the etymology. The "son" part of the name Gleason does not link it to the Scandanavian naming pratice.