test.js

var sassFilename = "style." + $('#sass-flavor :selected').prop('value');

var data = {
    "description": "posting gist test",
    "public": true,
    "files": {
        sassFilename: {
            "content": sassCode
        },
        "structure.html": {
            "content": htmlCode
        }
    }
}

var sassFilename = "style." + $('#sass-flavor :selected').prop('value');

var data = {
"description": "posting gist test",
"public": true,
"files": {
"structure.html": {
"content": htmlCode
}
}
};

data[sassFilename] = {content:sassCode}

Is what I think you want?