Glewlwyd first appears in the tenth-century Welsh poem Pa Gur yv y Porthaur, "What Man is the Gatekeeper?" preserved in the Black Book of Carmarthen, one of the earliest vernacular Arthurian works in existence and a text that alludes to lost adventures of Arthur and his men that no later chronicle ever recorded. In this fragmentary poem, Glewlwyd stands as the porter of a fortress and refuses entry to Arthur and his war band until the king proves his right to pass. Arthur responds with a roll call of his companions and their exploits: Mabon son of Modron, the servant of Uther Pendragon; Manawydan son of Llŷr, who bore pierced shields from Tryfrwyd; Anwas the Winged and Llwch the Windy-Handed, guardians of Edinburgh; and above all Cai the Fair, who would implore his enemies even as he slew them three at a time. The poem breaks off in the middle of a description of Cai's battle with the terrible Cat of Palug, but its significance is complete: Glewlwyd is the guardian who forces the hero to name himself and his deeds before the adventure can begin.

The defining encounter of Glewlwyd's life came in Culhwch and Olwen, the longest and earliest of the surviving native Welsh prose tales and arguably the first extended Arthurian story ever written. Culhwch ap Cilydd, a prince cursed by his stepmother to love only Olwen, the daughter of the giant Ysbaddaden, arrived at Arthur's court at Celliwig to claim his cousin's aid. Glewlwyd stood at the gate, massive and immovable, and refused the stranger entry. Culhwch threatened to wreak havoc unless he was admitted, but Glewlwyd did not flinch. It was Arthur himself who commanded that the door be opened, and only then did the porter stand aside. Once inside, Glewlwyd delivered a bombastic speech that has become one of the most famous passages in Welsh literature. He declared that two thirds of his life had passed alongside two thirds of Arthur's, and he listed the places he had been: Caer Se and Asse, Sach and Salach, Lotor and Ffotor, Greater India and Lesser India, the battle between the two Ynyrs when twelve hostages were brought from Norway, Europe and Africa, the islands of Corsica, Caer Brythwch and Brythach and Nerthach. He had been there when Arthur overcame the troops of Gleis son of Merin, when the king killed Mil the Black son of Dugum, when Arthur conquered Greece, in Caer Oeth and Anoeth and in Caer Nefenhyr. He had seen handsome noble-looking men in every corner of the world, he concluded, but he had never seen a man like the one who now stood at the entrance of the gate.

The character that emerges from this speech is not merely a porter but a veteran of impossible campaigns, a man who has travelled to the edges of the known world and beyond, to the Otherworld itself. Caer Oeth and Anoeth, the Fortress of Difficulty and Impossibility, were names of the Welsh Otherworld, and Nerthach was a place of supernatural power. Glewlwyd was not simply guarding a door; he was guarding the threshold between the mortal realm and the realm of marvels, and his qualifications for that post were written in the battles he had survived and the lands he had seen. When the giant Wrnach Gawr later sent his own gatekeeper to question Cai and Bedwyr, the episode echoed Glewlwyd's interrogation of Culhwch, suggesting that the gatekeeper motif was a recognised convention of Welsh heroic narrative, a ritual testing that every hero must pass before the adventure can truly begin.

Glewlwyd's physical presence was as formidable as his履历. The Welsh Triads, ancient mnemonic poems that catalogue the great figures of Welsh tradition, named him one of the Three Offensive Knights of Arthur's Court, alongside Sanddef the Beautiful and Morfran the Ugly. They were called offensive because it was repugnant to anyone to refuse them anything: Sanddef because of his beauty, Morfran because of his ugliness, and Glewlwyd because of his size, his strength, and his ferocity. He was further named as the lover of Dyfyr Golden-Hair, one of the Three Famous Maidens of Arthur's Court, a detail that hints at a romantic dimension to the warrior-porter that the main narratives do not explore. In the fifteenth or sixteenth century, a satirical Welsh poem called Araith Iolo Goch alluded to his strength in a passage of hyperbolic praise, describing him as the man who lifted a cauldron down from the fire with one hand in the court of Toron of the Three Islands of Britain, even though the sliced meat of seven oxen was boiling inside it.

In the later Welsh romances, Glewlwyd's role expanded and refined. In Geraint fab Erbin, he served as chief gatekeeper at Caerleon-on-Usk, though the text noted that he did not trouble himself with this service except on one of the three great festivals of the year. For the rest of the year, his duties were shared by seven assistants: Gryn, Penpingion, Llaesgymyn, Gogyfwlch, Gwrddnei Cat-Eye, Drem ap Dremidydd, and Clust fab Clusfeinydd. In Owain, or the Lady of the Fountain, he appeared again at Caerleon, greeting guests and foreigners, telling them the customs and habits of the court, and informing those who had the right to go to the hall or the chamber or who merited lodging. He had become, in these later texts, not merely a barrier but a guide, a figure who welcomed the worthy and directed them through the labyrinth of courtly protocol.

The hunt for Twrch Trwyth tested Glewlwyd as nothing else had. When Arthur and his host pursued the enchanted boar across Ireland and into Wales, Glewlwyd led his own band of servants in the chase. The boar was a creature of supernatural ferocity, with venom-dripping bristles and a comb, scissors, and razor lodged between its ears, and it killed men and hounds alike as it ran. Three of Glewlwyd's servants, Huandaw, Gogigwr, and Penpingion, were mortally wounded by the quarry and died. When the hunt ended and the survivors returned, Glewlwyd was left with only one servant, Llaesgymyn, a man described as being no use to anyone. The greatest porter in Britain, the veteran of a hundred battles, had been reduced by a single boar hunt to a master with one useless servant, a humbling that speaks to the terrible cost of the greatest hunt in Welsh story.