I'm quite new to GraphQL, I'd never even heard of subscriptions before!
After having now read a little about it, it seems like GraphQL uses a PubSub service to update clients. AWS has SNS for this purpose, so I'd imagine you could use that.
Looking around, seems there's an older project that might be a good base (although outdated): https://github.com/jonsharratt/graphql-aws.
Please do let us know how it goes