The object your method decompressGZIP() needs is a byte[].

So the basic, technical answer to the question you have asked is:

/** String to byte */

byte[] b = string.getBytes();
byte[] b = string.getBytes(Charset.forName("UTF-8"));
byte[] b = string.getBytes(StandardCharsets.UTF_8); // Java 7+ only

JSON to Byte

use org.alibaba.fastjson

   public static void main(String[] args) {
        List<String> msisdns = new ArrayList<>();
        msisdns.add("89860445101980684910");
        msisdns.add("89860445101980684903");

        MsisdnsTo msisdnsTo = new MsisdnsTo();
        msisdnsTo.setMsisdns(msisdns);
        System.out.println(msisdnsTo);
        System.err.println(JSON.toJSONString(msisdnsTo).getBytes());
    }