So I've reached an acceptable solution for compiling Sass via Gulp on Forge. Notes:

1. Use the gulp-sass package (not gulp-ruby-sass). Dealing with installing and keeping Ruby updated is not worth the effort to me. I rewrote my Sass to use Autoprefixer instead of Compass, and for sprites you should be using SVGs not compiled bitmaps at this point. It's a fair deal.

2. Since gulp-sass is based on node-sass and hence libsass, it is behind the Ruby implementation of Sass. This means Sass 3.3 is not supported and external source maps are not available (inline only). My application is small and I wrote all the CSS so working without source maps isn't a huge barrier.

3. I'm using the gulp-if and minimist package to detect environments and prevent Growl-style notifications from running on my server. Here's snippet of some of that code:

var gulp = require('gulp');

var autoprefixer = require('gulp-autoprefixer');
var cache        = require('gulp-cache');
var concat       = require('gulp-concat');
var del          = require('del');
var gulpif       = require('gulp-if');
var imagemin     = require('gulp-imagemin');
var jshint       = require('gulp-jshint');
var notify       = require('gulp-notify');
var plumber      = require('gulp-plumber');
var rename       = require('gulp-rename');
var sass         = require('gulp-sass');
var shell        = require('gulp-shell');
var svgmin       = require('gulp-svgmin');
var uglify       = require('gulp-uglify');

// ----------------------------------------------
// Environment Detection
// Run as "gulp --production" to prevent notifications on server.
// See: http://journal.toddsmithsalter.com/a-simple-gulp-build-task/

var flags        = require('minimist')(process.argv.slice(2));
var isProduction = flags.production || flags.prod || false;

// ==============================================
// Error Handling
// ==============================================

function handleError (err) {
    console.log(err.toString());
    this.emit('end');
}

// ==============================================
// Tasks
// ==============================================

gulp.task('default', ['clean'], function () {
    gulp.start('css');
});

// ----------------------------------------------
// Clean
// Delete old generated files.
// The "callback" ensures completion before proceeding to generation tasks.
// See: http://markgoodyear.com/2014/01/getting-started-with-gulp/

gulp.task('clean', function (callback) {
    del([
        '_themes/manuals/css/*.css',
        '_themes/manuals/js/build/*.js'
    ], callback);
});

// ----------------------------------------------
// CSS

gulp.task('css', function () {
    return gulp.src('assets/scss/**/*.scss')
        .pipe(plumber({ errorHandler: handleError }))
        .pipe(sass({
            outputStyle: 'compressed'
            //sourceComments: 'map'
        }))
        .pipe(autoprefixer({
            browsers: ['last 2 version', 'safari 6', 'ie >= 10', 'opera 12.1', 'ios 6', 'android 4']
        }))
        .pipe(gulp.dest('public/css'))
        .pipe(gulpif(!isProduction, notify({
            title: 'Gulp: CSS',
            message: 'Finished <%= file.relative %>'
        })));
});

Then edit your Forge deploy script to add:

gulp --production

So that's how I'm working with Sass + Gulp + Forge. It's not perfect but it works.