Gardel doesn't really know how the Cottonfoot family left Oerth, he just knows they were glad to do it. Something about a debt to a demigod warlord or something. It doesn't matter.
Papa Cottonfoot was an industrious fellow in Sigil, a textile-maker in charge of a pretty large shop. An Indep, too, didn't believe in taking favors from nobody and didn't believe in giving them out, either. An honorable man but too independent for his own good. Mama Cottonfoot, however, was a proud Sensate, dedicated to knowing and doing. The two clashed but there was real love in their hearts; or something like that. Life's more fun with a Sensate, and it's hard to experience new things if you can't afford them. Gardel was their second child, and hardly an appropriate heir; his sister Keila had a much better mind for business. An almost Infernal one, you could say.
From an early age, going to all of those Sensate meetings, Del learned how to play music and how to keep his pointy ears open and mouth shut. Not always the best thing to do, but he did it all the same. Soon he started writing his own words, based on what he saw and what he knew (if he could trust that), and started using that oft-ignored emotion of empathy to feel what others felt. He used this to his advantage, and sneaking away from one of his mother's more experiential sessions, he performed for the first time without any supervision at fourteen. It was fantastic, he got a decent crowd (four berks, one of whom was a deaf primer, but what can you do?) for a first gig, and he fell in love.
Unfortunately, that didn't go over well with Papa Cottonfoot. He wanted his son to be industrious. While Mamam Cottonfoot supported him, she would never raise a word against her husband, and that was that. By seventeen, he was out, living on the streets of Sigil on his own, moving through circles that rarely allowed him to cross over with his parents or his family (his twin younger brothers tended to take after Papa Cottonfoot as well, just less meticulously). On a good night, after a few drops of cherryfire from Toril, he realizes that his father was pushing him to work on his own, a real Indep, but on every other night he spits at the mention of anything remotely paternal. He hears his sister went independent as well, and is building a textile empire (and maybe some more questionable practices as well). He's more careful to avoid her than he is their father.
It's been a ride since then. Weddings that turned into bloodbaths. Playing background music for parties that were really meant to discuss the end of the world (well, some world; not his world). Doing beat poetry for the hottest concert this side of the Elemental Planes. He's done it all. Well, not all. He's still up and coming. People might have heard of him. He wants to be heard. He's been crafting his persona carefully, trying to find the real Gardel, the Gardel he wants Sigil to sing the praises of.
He just doesn't know who that is yet.