As far as I can tell, there is no way to achieve what is possible with the following REST APIs:
Events API - https://developer.github.com/v3/activity/events/
Notifications API - https://developer.github.com/v3/activity/notifications/
It’d be great to be able to access these via GraphQL because it’s a great fit for GraphQL strengths. Sometimes, you want to look up a few extra fields related to a notification or event, and with REST API, that requires additional requests. With GraphQL, in theory, one should be able to fetch everything one needs in one query.