We don't have any direct support of GitHub status updates. You can make whatever requests from within the build (presumably at the end), but we don't (yet!) have a supported way to get secrets into the build for access to POST the new status.
GCCB will also publish on Cloud Pub/Sub when the build's status changes (including when it goes to "SUCCESS" or "FAILURE"), and you can tell Cloud Pub/Sub to make POST requests with that data, but the format is not flexible so it might not work out for GitHub status updates.