At the dawn of the Age of Fire, Gwyn discovered a Lord Soul and thus became the Lord of Sunlight alongside three other Lords.
His Lord Soul granted him the power of the sun and lightning, and Gwyn helped defeat the Everlasting Dragons; his "mighty [lightning] bolts peeled apart their stone scales".
Thousands of years ago, the First Flame began to fade. The Witch of Izalith attempted to recreate the first flame, but failed and the flame engulfed and corrupted her and most of her followers, creating the Bed of Chaos and the Chaos Flame. Gwyn waged war against the demons at one point, but the attempt failed.
Gwyn was also known to hunt dragons and wyverns later on and had Four Knights. The dragon trophies can be found all around on the walls in Anor Londo
He bequeathed parts his Lord Soul to the Four Kings of New Londo, and Seath the Scaleless -as well as a dukedom to the latter.
Ultimately, Gwyn left Anor Londo to travel to the Kiln of the First Flame to rekindle it and thus prolong the Age of Fire. He brought half of his knights with him, while the rest stayed behind to guard Anor Londo. After successfully linking the fire, the First Flame burned Gwyn and his knights, reducing him to the Lord of Cinder, though his soul remained powerful.
Gwyn's army of knights that accompanied him to the Flame were transformed into by the Chaos Flame, becoming the Black Knights.
Gwyn's friend, Havel the Rock, was a general over his own warriors, which were presumably part of Gwyn's army.