Two of the most prevalent symbols in Sir Gawain and the Green Knight are the pentangle and the girdle. Like many symbols, the meaning behind each have changed over time.
In the tale, the pentangle symbolizes the one thing that keep Sir Gawain focused upon his quest. The pentangle (which depicts the Virgin Mary on one side and the five fifths on the other) gives Gawain strength. (The five fifths represent the five fingers, five joys, five senses, five wounds of Christ, and the fifth five (friendship, fraternity, purity, politeness, and pity).)
Today, the pentangle (otherwise known as the pentagram) has changed. Today, the symbol represents one of evil. While, historically (as represented in the text) the pentangle did represent Christianity, the symbol has been adopted by occult followers and is seen as a symbol associated with Satanism. Therefore, the relevance does not hold, universally, as it has in the past.
As for the girdle, the girdle (in the text) represented Sir Gawain's sin. It became representative of his own personal badge of dishonor. Upon his return to Arthur's castle, Arthur enforced a new law where all under his control would wear a band of green to symbolize Gawain's honor.
Today, the girdle represents many different things. For some, the girdle represents the hiding of reality. Women wear a girdle to cinch in their waists and show their bodies to be something that they are not. Therefore, the girdle hides the truth. Not worn in contemporary times as Gawain did, the girdle is hidden under clothes and not meant to be seen.