The Greek god Chaos (also spelled Khaos) was, in fact, a personification of the void, of the empty space In his Theogony, Hesiod says: "In truth at first Chaos came to be...". For him, Chaos was "a gaping chasm". It is not known whether it was created or it had always existed. Out of it all the gods, the men and the things arose.
Immediately after Chaos, the "wide-bosomed Gaia" (Earth) appeared - we don't know whether she appeared by herself or was the offspring of Chaos. Hesiod also says that the children of Chaos are Erebus (the personification of darkness and shadow) and Nyx (the goddess of night).
It is Ovid, in his Metamorphoses, who calls Chaos "a rough, unordered, mass of things" and this is the meaning of the word "chaos" that we know today. It's clear then why we can't answer to the question: "How did the greek god Chaos look?"
In this painting we can see a possible representation of the primordial Chaos:
And another one by... that's a tough one, really! On the Allposters site, the name is indicated as "Briout". I have looked for him on the Internet and it was difficult to find the name as such. It seems the real name is Isaac Briot, a French engraver and medallist who lived between 1585-1670 and made some of the illustrations for a French version of Ovid's Metamorphoses.