When the lady tells Gawain that the sash is a talisman that will protect him,he does not doubt her. Does his belief in its power reveal him as naive andgullible?

I think if we examine the play within the context of the period, it's not such a stretch. Consider that wizards like Merlin were kicking around as well as the occult arts. Gawain is also a guy that believes in honour until the person proves dishonourable.