GatlingResponseBody.scala

    .exec(http("Goto Your Desired URL")
      .get("http://THIS_SHOULD_BE_YOUR_URL")
      .check(
        status.is(200),
        bodyString.saveAs("responseBody")
      )
    )
    .exec(
      session => {
        // You can do whatever you want with the string
        var htmlText = session( "responseBody" ).as[String]
        var index = htmlText.indexOfSlice("SOME_SEARCH_VALUE")
        var TXNO = htmlText.substring( index, index + 16 )
        println( "TXNNO in SESSION: " + TXNO )
        // Set the output back to session
        session.set( "TXNNO", TXNO )
      }
    )