I know I just said that Gogo is meant to be a non-sequitur.

But it got me thinking; what purpose does Gogo serve in the story? What is he?

He's a mime. He dresses in green and red and yellow...but he's totally self-effacing, not a hint of ego or passion about him. Nonchalant.

Is Gogo the antithesis of Kefka?

That got me started thinking, "What if when Kefka absorbed the Triad's power, the last remaining vestige of his humanity was separated from him somehow, became self-aware, but without any of Kefka's experience memory?"

Gogo.

Why does he wind up on Triangle Island?

Triangle Island is a locus of power for the Triad in the Human World.

You fight Intangirs there, which are not only tough as balls and cast Meteo, but they give 10 Magic Points when you beat them.

Unfortunately, the Zone Eaters were unearthed in the collapse, so when Gogo materialized on Triangle Island, he eventually ran into one. With no one to Mimic, he was sucked into the Zone Eater.

How did he get to the bottom without being assraped?

...Some things are just Gameplay and Story Segregation.