A malicious knight in Heinrich von dem Türlin’s Diu Crône.
He killed the brother of Sir Aamanz, who was called ‘the Other Gawain’ due to his striking similarity to Arthur’s nephew. Aamanz pursued Gigamec in revenge, and the two knights encountered Gawaine himself.
In a confusing melee, Gawaine defeated Aamanz and gave him to Gigamec, who, with another knight named Zedoech, murdered ‘the Other Gawain’ as soon as the real Gawaine had left. Gigamec then brought Aamanz’s head to Arthur’s court, representing it as Gawaine’s, causing great distress until the truth was discovered.
Source
Diu Crône | Heinrich von dem Türlin, c. 1230