Recreate new field from old field

Elastic Stack Logstash
1

I have used add_filed, remove_field, rename etc.. many time over on logstash

now I have something different that I can't quite figure out.

runnign nvidiabeat and it gives me following

{
        "clocks":{"mem":405,"sm":139,"gr":139},
        "memory":{"used":623,"total":11172},
         "utilization":{"gpu":0,"memory":0},
        "pstate":8,
        "power":{"limit":135,"draw":16.38},
        "driver_version":384.9,
        "fan":{"speed":23},
        "type":"nvidiagpubeat",
        "count":4,
        "name":"GeForceGTX1080Ti",
        "temperature":{"gpu":30}
 }

I want to move whole thing under

"system": {
    "gpu": {
             "clocks":{"mem":405,"sm":139,"gr":139},
            "memory":{"used":623,"total":11172},
             "utilization":{"gpu":0,"memory":0},
            "pstate":8,
            "power":{"limit":135,"draw":16.38},
            "driver_version":384.9,
            "fan":{"speed":23},
            "type":"nvidiagpubeat",
            "count":4,
            "name":"GeForceGTX1080Ti",
            "temperature":{"gpu":30}
   }
}

I have try this as test but didn't work

 if [utilization][gpu]  {
        mutate { add_field => {"[system][gpu][utilization][gpu]" => "%{[utilization][gpu]}"  }
     }
  }
2

well it is working now.
I had this inside some other loop and hence it was not even picking up.

consider it as solved.