I believe the answer is that Sir Gawain thought the Green Knight belonged to the nobility, because in the excerpt it says that 'the fabric was noble' - meaning that not everyone (such as peasant, vassal, or commoner) could afford such an attire.
The attire of Bertilak, the mysterious Green Knight who challenges the knights of the Round Table, is described in the medieval text in detail in lines 161-172, where we can read that "Bothe the bosses on his belt and other bright gems / That were richly ranged on his raiment noble". The word noble indicates indeed his social status; it grants him his high position in the court. Additionally, the fact that precious stones and metals are part of his attire also reflects the lavishness of the royalty and the court.
