Possibly due to the ordeals he suffered, is skin was covered in barnacles, and his left arm was replaced by a large mechanical crab claw. He wielded a magical trident known as Drown, one of the four elemental tools made by Vizeran DeVir.
His backstory is revealed in the adventure book, Princes of the Apocalypse.
Gar was born in a poor fishing village in the Nelanther Isles. When he was young, sahuagin attacked and slaughtered everyone in the village; soon after, he was conscripted by a Tethyrian merchant galley and grew up in brutal indentured servitude that was little more than slavery. When the ship was destroyed by pirates, he clung to the wreckage for days until a shark ripped off his left arm. He finally gave up all hope and accepted his imminent death, but a strong current carried him away from the predator to the shores of the Sword Coast.
Believing that the sea itself had saved his life, he spent the next several years devoting himself to studying the ocean, learning powerful water magic. His harsh experiences with both the mercenaries and pirates led him to distrust other people, but he came to believe that it was his calling to find others like him and teach them the power of the sea.
When he began to receive visions from the Elder Elemental Eye, he followed them to a hidden cavern beneath the Sumber Hills where he found a powerful magic trident called Drown imbued with the power of the primordial Olhydra, Princess of Evil Water. Taking up the trident, he devoted himself to Olhydra and expanded his followers into the Cult of the Crushing Wave.
All their work culminated in 1491 DR at in the Fane of the Eye, under the ruined dwarven city of Tyar-Besil. Under their manipulation, a structure known as the Fountain of Delights was build at the Raven Cliff Beach. A huge tidal wave then swept the beach, forming the Drowned Shore. Working simultaneously with the other three elemental cults, the water cultists tried to summon Olhydra to Toril using members of a missing diplomatic delegation from Mirabar and other captives as sacrifices, mostly aquatic elves.
However, the problem soon comes to known in Neverwinter. When Archdruid Morningdawn of the Emerald Enclave arrived in Neverwinter with word of a new threat rising in the Sword Coast, Lord Neverember took it seriously. With his permission, she erected the Tree of Elemental Balance to guard the city of Neverwinter. She explained that the currents of elemental forces had changed in such a way to cause dangerous concentrations of the elements, and that the tree would allow her to stabilize the flow. However, someone or something was interfering.
The Emerald Enclave had reporting a building pressure of earth and water in the Ebon Downs, and while the great warrior Minsc had volunteered to investigate Morningdawn believed that more than his zeal would be needed and asked the Heroes of Sleeping Dragon Bridge to go with him. Deep in a cave within the Ebon Downs, Minsc and the heroes found cultists of elemental earth and water creating concentrated manifestations of elemental energy known as elemental seeds. When they brought the earth and water seeds back to Morningdawn, she concluded that the disturbance she felt was due to these elemental cults, who were apparently trying to create elemental nodes - two-way conduits to one the elemental planes. Later, Minsc and the adventurers foil the fire and air culitists' plan in Pirates' Skyhold and received the fire and air seed.
Fraunstrated for their failure, the four prophets gathered their army and attacked Neverwinter in an attempt to destroy the Tree of Elemental Balance. Gar's army was the first arrived, which included cultists, water archons, sea trolls and sea hags. After their defeat, the prophets had a discussion. Gar argued bitterly with Vanifer, but in the end, they accepted Aerisi Kalinoth's suggestions, and the four retreated back to their respective fortresses to bulid elemental nodes. Meanwhile, Archdruid Morningdawn tasked the heroes with eradicating the cults' presence along the Sword Coast.
The heroes arrived at the Drowned Shore and allied with Harpers, the Zhentarim assasins and Emerald Enclave members there. Together, they foiled Gar's plan as he tried to sacrifise an aquatic elf to Ollydra, apparently killing him. After the elf was saved, she revealed that her sacrifice to Olhydra would have appeased the primordial enough to grant Gar a devastation orb, with which the cult could have created another tidal wave. In saving her, the heroes had in truth saved thousands.