According to Diu Crône, a knight whose nickname was 'the other Gawain', due to his uncanny physical similarity to Sir Gawain.
He was in purusit of a knight called Gigamec, who had slain his brother. Another knight named Zedoech intercepted and challenged him. Sir Gawain happened to come along and stopped Aamanz as he was about to behead his opponent. The two battled and Gawain won, but Aamanz refused to surrender, which caused Gawain to turn Aamanz over to Zedoech and Gigamec, who murdered him as soon as Gawain had left.
Gigamec then brought Aamanz head to Arthur's court, representing it as Gawain's, which caused great distress and confusion.