Thenne watz Gryngolet grayþe þat gret watz and huge
and hade ben soiourned sauerly and in a siker wyse.
Hym lyst prik for poynt þat proude hors.
—Sir Gawain and the Green Knight, 2047-9.
In the medieval tales of King Arthur and his knights of the Round Table, "Gringolet" was the loyal steed of Sir Gawain, who was either Arthur's greatest knight (if you believe the English), or second-greatest knight after Sir Lancelot (if you believe the French).
Gringolet is a male horse, and is almost always described as a charger, in other words, the generic term for a horse specifically bred and trained for use in mounted combat. When a color is given for Gringolet, he is usually described as pure white in color, which is an obvious symbol of Sir Gawain's purity as a knight.
Gringolet's earliest known appearance as the name of Gawain's horse is in Chrétien de Troyes' late-12th century French romance Erec and Enide, in which Gawain loans Gringolet to Sir Kay to joust against Erec, although even Gringolet's might cannot save the incompetent Sir Kay from defeat at the hands of Erec, who after all is the hero of that tale. Chrétien's romances were incredibly influential, and "Gringolet" became the canonical name for Sir Gawain's horse in all later Arthurian romances.
In the romances, there was no consensus on Gringolet's origin story. In the Vulgate Cycle, Sir Gawain wins him in a contest with a Saxon warrior, but in Wolfram von Eschenbach's German romance Parzival, Sir Gawain discovers him in the sacred stables of the Grail Castle.
The name "Gringolet" is of unknown origin as well. Many of the French names for Arthurian characters were derived from earlier Celtic forms, so it may have been that the name "Gringolet" derived from some sort of pre-Chrétien Brythonic tradition now lost, and in light of this possibility scholars have suggested two possible derivations from Welsh - gwyn caled meaning "white hardy" or cein caled meaning "handsome hardy."
It is equally possible, however, that Chrétien simply made up on his own what he felt was a really cool-sounding name for a horse.