Pylon is an Open Source API Gateway which is written in Elixir.
It uses a redis backend.
In a previous life (job) I worked on an erlang adserving application. It was basically a giant API Gateway. We implemented features such as rate limiting, traffic mirroring, and fetching from multiple sources and aggregating the result.
Erlang was an amazing language to do these sorts of tasks. We could decide what tasks needed to happen in real-time, while others could happen as background tasks. Erlang handles this very nicely as well as handling a MASSIVE amount of concurrency.