See new centralized resource created by Wade. Note this is github, you can contribute via pull requests ;-).
developer server: http://hedwig.bmi.stonybrook.edu
Joe also made a pitch for https://about.sourcegraph.com
Elegant intro to coding it https://www.youtube.com/watch?v=Fnwj9VjE_BY, includes example that uses Express.
It helps understand Linux and git, coding things that work: https://www.youtube.com/watch?v=o8NPllzkFhE
Messaging with full Web features (you can message an application by invoking the JS that once injected will assemble it) changes everything: NYT Video.
report from yesterday: https://cloudplatformonline.com/summit-NewYork-2017.html
If I understood your question correctly you are trying to use Javascript to invoke a webhook to perform an action, i.e. interact with GitHub.
If so, I believe Cloud Functions would be a great solution for you...
https://cloud.google.com/functions/
This is more specific, HTTP Triggers...
https://cloud.google.com/functions/docs/calling/http
If you are simply looking to update Github via Google Cloud, this will synchronize your GitHub and Cloud Repo...
https://cloud.google.com/source-repositories/docs/connecting-hosted-repositories