You might consider duplicating Gawain's shield emblem, which is a pentangle, a five-pointed star, with the Latin motto Quinque virtutes, una culpa, loosely translated as "Five virtues, one fault or sin."
One of the important themes of Sir Gawain and the Green Knight is Gawain's perfect behavior as a knight (and knightly behavior, in general). His entire journey to find the Green Knight, including the temptations he faces at Bercilak's court from Lady Bercilak, which he manages to avoid while not violating his obligation to be polite, is one of hardship and loneliness:
Except for his horse, he had no company in the forest and hills/during this journey only Christ to talk to (ll. 695-96)
The Pearl-Poet is constantly reminding the reader of Gawain's virtues in the face of adverse terrain and events and, more important, that Gawain is intensely devoted to Christ and looks to Christ for solace in the face of adversity, as one expects of a perfect Christian knight of Arthur's court.
Because Gawain's virtues are always in groups of five, represented by the Pentangle on his shield (e. g., the five wounds of Christ, Gawain's five knightly virtues), using a motto that acknowledges the significance of five makes sense. More important, because Gawain has only one failure--his lack of confidence in his own virtues, expressed by his use of the garter from Lady Bercilak--a reference to that failure brings attention to one of the most important themes in the poem--the momentary failure of belief.