isnapp

You also need to set the correct host

define('DB_HOST', ':/cloudsql/your-app-id:sql');

Replace the your-app-id:sql with your own app-id and Cloud SQL instance name.

I suspect that you don’t have wordpress in a subfolder wordpress inside your app folder.
Because of that wordpress could have an issue finding the wp-config.php

If by running from the webroot you mean that you don’t see an extra /wordpress/ in the url on your browser then you can use the yaml below:

application: your-application-id
version: wp
runtime: php
api_version: 1

handlers:
- url: /(.*\.(htm|html|css|js))$
  static_files: wordpress/\1
  upload: wordpress/.*\.(htm|html|css|js)
  application_readable: true

- url: /wp-content/(.*\.(ico|jpg|png|gif))$
  static_files: wordpress/wp-content/\1
  upload: wordpress/wp-content/.*\.(ico|jpg|png|gif)
  application_readable: true

- url: /(.*\.(ico|jpg|png|gif))$
  static_files: wordpress/\1
  upload: wordpress/.*\.(ico|jpg|png|gif)

- url: /wp-admin/(.+)
  script: wordpress/wp-admin/\1
  secure: always

- url: /wp-admin/
  script: wordpress/wp-admin/index.php
  secure: always

- url: /wp-login.php
  script: wordpress/wp-login.php
  secure: always

- url: /wp-cron.php
  script: wordpress/wp-cron.php
  login: admin

- url: /xmlrpc.php
  script: wordpress/xmlrpc.php

- url: /wp-(.+).php
  script: wordpress/wp-\1.php

- url: /(.+)?/?
  script: wordpress/index.php

This is exactly the app.yaml from https://developers.google.com/appengine/articles/wordpress
but with the trailing $ removed from (just) the upload lines.

For me this works as a charm.

Removing the $ at the end of the upload line(s) should make it work.

So for example change:

upload: wordpress/.*\.(htm|html|css|js)$

to

upload: wordpress/.*\.(htm|html|css|js)