Ah, very cool.
I actually wrote most all of GC full-stack, minus the frontend app for creator-editor and reader, which is a standalone React app.
The rest of the stack is PHP (HHVM actually), MySQL (Maria DB), REDIS/MemCached (dual caching layer), GearMan (asynch processes), Minio for bucket store (Amazon S3 open source clone), and Twig Templates for non-react frontend.
It is 100% custom built, including a custom PHP framework my friend and I wrote back in 2010 The stack itself is fully containerized in Docker containers enabling micro-service deployments and upgrades.
We also use an ELK stack for logging/monitoring, along with sentry and some other misc stuff.