Eventually he ended up helping a castle lift a siege, that it just so happened was being laid by his uncle Ector and Gawaine. With the magic Sword of Avelion he dealt Gawaine a head wound that would leave him bed-ridden for a month, and then disappeared without even saying hello.
Eventually, near Castle Corbin, in the hermitage of Sir Ulfin, he was summoned by a mysterious gentlewoman (who later turned out to be Percivale's sister) and led to the seaside, where they both joined Percivale and Bors aboard the priest's boat, and set sail away from Logris.
Only Galahad succeeded in drawing the Sword of David from its cruddy hemp scabbard, and Percivale's sister gave him new girdles woven from her own hair and gold, and renamed it the Sword with the Strange Girdles, and named the scabbard Mover of Blood. Destiny fulfilled, they returned to the priest's boat and sailed away, leaving Solomon's magic ship to look after itself.
Then they followed a white hart and four lions that turned into two men, another lion, an eagle and an ox that passed through a glass window without breaking it in some peculiar allegory of the anunciation.
They set her corpse adrift in a barge clutching a letter of explanation, and during the following thunderstorm Bors rode off to save a passing knight from another knight and his dwarf. When Galahad and Percivale returned to the Castle of Blood they found everyone they hadn't killed earlier was now dead by the storm, and exploring found the tombs of all the previously drained maidens.
Then they went their separate ways, sobbing.
After twenty-four days in a coma (one for every year he had been poking Guenever) he woke up and there was King Pelles, who told him that he had seen as much of the Sangreal as he was ever going to, and that his daughter Elaine (who had ravished Launcelot and born Galahad) was now dead.
Four days later Launcelot's brother Ector rode up and knocked on the door, but the castle would not let him in, and so he rode off in despair. The next morning, Launcelot left himself, for Camelot.
When he finally returned to Camelot, a year after having left it on the quest, he found Arthur and Guenever well, but fully half of the 150 round table knights dead or still missing, including Galahad, Percivale and Bors. Amongst those already returned safely were his brother Ector, and Gawaine and Lionel.
After this, Mallory gets very confused the narrative implodes somewhat. It becomes highly uncertain as to exactly who is the Maimed King, what sword goes where, exactly what the Sangreal is and how anyone can tell when it is eventually achieved. Several cursory and puzzling references are made to events that that may or may not have been introduced earlier in the narrative. It is as if Mallory was getting bored trying to make sense of his huge pile of assorted myth bits and just chucked everything that was still left into the stir-fry and turned up the heat.
Then he rode on to the White Abbey to gawp at Bagdemagus' tomb and extinguish the flames in the tomb of Simeon where his dad had failed at some point, and possibly another one too, before riding on.
Then the Maimed King was carried in, Pelles and Eliazar left (which would make the Maimed King the old King Pellam wounded by Balin with his own spear in book 2, and not a mis-print of King Pelles at all), and Joseph of Aramathie appeared dressed as a bishop.
There followed a major tripped-out scene heavily based on the Christian mass, involving glowing babies turning into bread people. Then Joseph left and Jesus came out of the "holy vessel" in person and Galahad "received his saviour".
After a mission briefing for his twelve new disciples Jesus declared that the Sangreal would now leave Logris never to return, blessed them all and vanished, leaving behind some of his blood on the Spear of Longinus.
Galahad did as he was told and used the spear to heal the Maimed King who then became a white monk.
They sailed far away to the city of Sarras (where, long ago, Joseph of Aramathie had converted King Evelake to Christianity), where they conveniently found the boat with Percivale's dead sister aboard, healed a passing cripple and buried sis. But then the evil pagan king Estorause threw them all in jail.
A year later he felt ill, got them out, asked forgiveness and died, and the people made Galahad king (instead of killing him).
Then, realising how far he was from home, he found a ship and sailed back to Logris.
United in their grief over the death of Galahad, Bors and his uncle Launcelot got all huggy and swore eternal friendship. And that was the end of the quest for the Sangreal.