Here’s an example how to use “Call REST” Activity for an GraphQL API. It uses a small custom java action (GraphQL query template 2 JSON) to transform a GraphQL query into a JSON query that can be Posted to a GraphQL endpoint.
You can call a GraphQL endpoint in the same way you’d call a regular REST endpoint. However, at this point Mendix doesn’t offer any assistance in building GraphQL queries, so you’d have to create the request body yourself and parse the response accordingly.