Getting an app that's built using gulp up and running on a public url on heroku can be a bit tricky. After messing with build packs trying to get my app to build in the cloud, I found a work around.
gulp build locally, a version of the app is built into dist/ folder within my project. This for me was a static site, and this can be run on heroku using a one line 'include' statement in php. Simply add an index.php file to the built version, and rename the index.html to home.html, then use the include statement, as shown here.