Best How To :

The complete working setup

package.json

"devDependencies": {
    "gulp": "~3.8.10",
    "gulp-sass": "~1.3.2", //this installs libsass for us
    "gulp-sourcemaps": "~1.3.0",
    "gulp-plumber": "~0.6.6",
    "susy":"~2.2.2"
}

Gulp sass Task

var gulp = require('gulp');
var sass = require('gulp-sass');
var handleErrors = require('../util/handleErrors');
var sourcemaps = require('gulp-sourcemaps');
var minifyCSS = require('gulp-minify-css');

//libsass
gulp.task('sass', function () {
    return gulp.src('./frontend/sass/style.scss')
        .pipe(sourcemaps.init())
        .pipe(sass({
            includePaths: [
                './node_modules/susy/sass' //required for sass
            ]
        }))
        //.pipe(minifyCSS()) //move to prod settings
        .pipe(sourcemaps.write('./app/www/css')))
        .pipe(gulp.dest('./app/www/css'))
});

Sass

@import "susy";

$susy: (
    global-box-sizing: border-box,
    use-custom: (rem: true),
    columns: 12,
    gutters: (1/4),
    debug: (
        image: hide,
        color: rgba(#66f, .25),
        output: background,
        toggle: top right,
    )
);

Errors

You will need to do the following to avoid the may only compare numbers error when running the task

Update node-sass's package.json

I had to update libsass's dependency as follows:

gulp-sass 1.3.1 
  ¬ node-sass 2.0.0-beta
     ¬ libsass 3.1.0-beta - This needed updating to libsass 3.1.0-beta.2

The above will become redundant as soon as node-sass update their dependency to use the newest libsass

Last Step

Then inside my sass use (1/4) rather than just 1/4. This will force it to be treated as a number. Bug taken from https://github.com/sass/libsass/issues/590

You should be able to run gulp sass without any errors.