He has glittering, yellow eyes, and while they are never shown, it can be assumed that he has large wings and a single row of sharp teeth.
He was found by Valkyrie and Skullduygery after they fall into a seemingly bottomless pit. He was , at first, believed to be a simple light, before the Gordon's head burst out of the darkness, accompanied by the distinctive flapping of large wings.
Valkyrie was incredibly excited by the fact that dragons really do exist, even quoting the title of a 2011 film, called There be Dragons.
And thus, she dubbed the dragon "Gordon", out of respect for the loss of the echo-stone recording of her late uncle.