This arrogant giant took his name from a Biblical reference which means 'all the elements of the earth'. Presumably he thought his strenght equalled that of all other fighting men.
In ancient times he was one of the original inhabitants of Britan, he led twenty others, protecting Albion against all invaders until the advent of Brutus the Trojan when he landed at Totnes, Devon. Brutus, who was exiled from Troy for accidentally killing his father, allied himself with another Trojan exile, Corineus. They and their men set sail for Albion to conquer it for themselves. When Gogmagog heard that these adventurers had landed he rallied his giant followers and they pounded across country to repel the invaders. They sighted the Trojans near Totnes, and without hesitation bellowed their thunderous warcries as they galloped to the attack.
But Corineus stepped forward to meet Gogmagog, dodged his whirling club, struck off his head and threw the great body over a cliff, Plymouth Hoe. The other giants immediately broke and ran, to vanish into deep caves from which they have never emerged.
Brutus soon conquered Albion and changed his name to Britain. He rewarded Corineus with a present of the western portion, which he named Cornwall in honour of the giantkiller.
His name possibly derives from the Bible (Ezekiel 38; passim; Revelation 20:8), and may probably be considered as a depaganisation by later Christian writers.
Gog and Magog | The Legend of King Arthur