Just use Java interop: https://github.com/intuit/karate#calling-java
You need to write a helper (start with a static method) to convert JSON to Avro and vice versa. I know teams using this for gRPC. Read this thread for tips: https://github.com/intuit/karate/issues/412
Also there is even a “karate-grpc” project: https://github.com/pecker-io/karate-grpc
Also see: