Arthur is renowned for his military career, there are even songs about his greatest exploit - the Battle of Red Rock.
Becoming the Lord
When Arthur was born, the Gully's were not the major house in the region of the Quell Sea, and were in fact a house in decline living in the meagre holding of Northpond. The only son to his fat, lazy father Meryn Gully, Arthur's mother died during his birth. Early in his days Arthur became a student of history, both from his father and the many scholars in the Gully court, he learned from an early age that the Gully family had once been a powerful and feared house, but over a period of many years had fallen into decline under lords who took their positions for granted.
This knowledge bred a great contempt in Arthur for his father, and an immense desire to see the Gully family restored to their rightful place as Wardens of the Quell Sea region. At this time, the Gully family had few bannermen, only the Moors were truly loyal to them, while their other bannermen, the Firemasons, had begun to grow discontent under the lazy leadership of Meryn. The Wardens of the region at this point was House Malver. House Malver had grown discontent with only holding control over the Quell Sea, and had been in great tension with House Ainsworth for many years.
Eventually, while Arthur's father was visiting with House Malver, they declared a rebellion against the crown, and demanded Arthur lead all of the Gully forces to Serpent's Roost, so they could join in the rebellion. Refusing to follow such a demand, Lord Carth Malver executed Meryn in the hopes it would bring Arthur to heel. In reality, Meryn's execution had been exactly what Arthur hoped for, and he took over as Lord of House Gully.
The Malver Rebellion
House Gully was weaker than the Malver's in terms of their army, but they had a greater navy, having been the major boat-builders for the Malver's during their time of servitude. Some of their armed forces were emboldened by retainers sent by the crown for their loyalty, but Arthur did not have a force strong enough to beat the Malver's conventionally, and certainly not enough to besiege Serpent's Roost. However, Arthur knew very well that the Malver's were aware of the Gully's weaker forces, and he counted on it.
Beginning a march toward Serpent's Roost, Arthur sent the majority of the Gully navy to distract what ships the Malver's did have in the Quell Sea itself, while a host of ships moved with his marching army along the cliffs, eventually meeting it at the White Rock. White Rock was an area right on the edge of the sea with high, cliffs of white stone and jagged rocks at its bottom. There was only two ways to approach the rock, from an easily defensible plain leading up a steep hill, or a land bridge, hidden by the tall rock itself which gave the area its name.
Arthur knew well about the land bridge, and although he figured the Malver's would use it he made no defences there, setting up all of his trenches and defenses on the plain, perfectly setting the guise of a camp waiting for a main assault. This emboldened Lord Malver, who sent a small force to distract the Gully's on the plain while the majority crossed the land bridge. When they were far enough across, Arthur had the catapults mounted on his ships open fire on the bridge, destroying it with ease and sending the majority of the Malver army falling onto the jagged rocks below. As the songs put it, the blood of the Malver's stained the White Rock red.
Easily wiping out the remainder of the Malver forces, Arthur marched on Serpent's Roost and took it relatively unopposed, putting every single Malver to the sword and wiping out the house entirely. For his service, the crown named house Gully the Wardens of the Quell Sea, rewarded Arthur with Serpent's Roost, and a bride, Lady Amara Moor.
Family
Settling into his life in Serpent's Roost, Arthur grew fond of his wife rather rapidly. He found himself reflected in her own ambition, and even her distaste for the Ainsworths, it reminded him much of his own hatred for the Malver family. The two grew to love each other despite any differences, and began to raise a family together.
Despite difficulties in the form of stillborn children which brought Arthur a great deal of frustration, he was never cruel with his wife, and continued to try for children with her despite an occasional discussion over whether it was worth it. Until this point Arthur had devoted too much of his life to his family name to see it crumble now. This frustration caused him to involve himself with a whore during a visit to the Diovire Desert, an experience he rapidly regretted, and chose to keep secret after returning to the Quell Sea. Eventually, their efforts bore fruit and their eldest son Lucan was brought into the world. A few years later, their second son was born, the pair of boys initially brought Arthur great joy, as he personally helped to educate and train them in the ways of history, war, and nobility.
Some years later, Arthur was confronted by his own bastard, Brandt, when the boy claimed to be his son Arthur was more than aware of its truth, he could see it in the boys eyes, but he refused to accept him. Devoted to his wife and regretting deeply even his involvement with Brandt's mother, he openly renounced any connection to Brandt and sent him away. This encounter awoke a more bitter side to Arthur, a contempt for himself which spilled over to his children. He became more harsh with training and teaching them, as if to wipe away his own stain of infidelity, he sought to further enrich the name of House Gully.
This contempt was only strengthened when his eldest son and heir renounced his titles to join the Kingsguard, it was only at the behest of his wife that he did not disown Lucan entirely, and he conceded later that he still held a great love for Lucan, despite his renouncement. Turning to his second son Ithobal, Arthur sought to mould him into the perfect lord, but it was not easy either. Granting him the gift of the Leviathan in an attempt to endear him to his father's word, Ithobal instead took to the open ocean to begin trading and sailing, an action which only built anger within the Gully Lord.
Out of all his children, only one commanded his respect, though still not his trust, and still with a degree of contempt - his daughter. Arthur has always been aware of his daughter's sly and treacherous nature, partly he believes it to be a result of his own teaching. He respects his daughter's ability to play the game, and as a result has granted her a position as his advisor, though he does not trust her, as he likely should not, and is not against using her for his own gain and efforts, regardless of what her own will may be.
For Arthur there is nothing greater than the family, he cares little for the wants and whims of his children.