Gideon was presented as Scrooge McDuck's younger brother. He has been appearing in many Italian stories as the editor of the newspaper "The Cricket", the most credible in Duckburg. He also has an antagonistic relationship with his brother.
His existence doesn't agree with Carl Barks's view of Scrooge as the last of The Clan McDuck. Though Don Rosa hasn't used him in his family tree or his timelines it has been purposed that he could be an illegitimate son of Fergus McDuck born somewhere between the death of Fergus' wife on 1897 and Fergus' own death on 1902. That would mean that Scrooge is the last legitimate McDuck. Gideon is now considered the last of Fergus' five children and a younger half-brother of Rumpus McFowl, Scrooge McDuck, Matilda McDuck and Hortense McDuck.