Nevermind, I figured it out!
Using PHP for GAE, app.yaml needs to specify the extensions ttf, svg, etc. as static files, otherwise they won’t be uploaded properly. I had to add |eot$|svg$|ttf$|woff$ in app.yaml as per below:
- url: /wp-content/(.*\.(ico$|jpg$|png$|gif$|eot$|svg$|ttf$|woff$))
static_files: wordpress/wp-content/\1
upload: wordpress/wp-content/(.*\.(ico$|jpg$|png$|gif$|eot$|svg$|ttf$|woff$))
application_readable: true
