Janus Graph Export and Import using GraphJson

Question

I am using the Gremlin I/O library ( GraphSONMapper + GraphSONWriter ) to export already existing Janus graph to a GraphJson.

The json file content after export is (export.json)
{
  "id": {
    "@type": "g:Int64",
    "@value": 2289864
  },
  "label": "user",
  "outE": {
    "user_group": [
      {
        "id": {
          "@type": "janusgraph:RelationIdentifier",
          "@value": {
            "relationId": "5po9l-1d2vc-1ij9-9mw"
          }
        },
        "inV": {
          "@type": "g:Int64",
          "@value": 12488
        },
        "properties": {
          "access_type": "E"
        }
      }
    ]
  },
  "properties": {
    "username": [
      {
        "id": {
          "@type": "janusgraph:RelationIdentifier",
          "@value": {
            "relationId": "5pnvd-1d2vc-2txh"
          }
        },
        "value": "tester"
      }
    ]
  }
}  

I want to import this json data to a new graph directly.I tried importing with the below command but facing below error in deserialization.

graph = TinkerGraph.open();
graph.io(graphson()).readGraph('/tmp/export.json');



Exception : org.apache.tinkerpop.shaded.jackson.databind.JsonMappingException: Could not deserialize the JSON value as required. Nested exception:org.apache.tinkerpop.shaded.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_OBJECT token
     at [Source: (ByteArrayInputStream); line: 1, column: 137]
     at [Source: (ByteArrayInputStream); line: 1, column: 137] (through reference chain: java.util.LinkedHashMap["outE"]>java.util.LinkedHashMap["user_group"]>java.util.ArrayList[0]>java.util.LinkedHashMap["id"]) 

Can someone please help me to import the above GraphJson to a new Janus graph with the same set of vertices and edges?