Gson gson = new Gson();
Type collectionType = new TypeToken<Collection<itemModel>>(){}.getType();
Collection<itemModel> objects = gson.fromJson(jsonString, collectionType);
// jsonString = "[{"name":"A","number":25},{"name":"B","number":26}]"
String filename = "export.csv";
FacesContext fc = FacesContext.getCurrentInstance();
ExternalContext ec = fc.getExternalContext();
ec.responseReset();
ec.setResponseContentType("text/comma-separated-values");
ec.setResponseHeader("Content-Disposition", "attachment; filename="" + filename + """);
OutputStream output = ec.getResponseOutputStream();

List<String> strings = new ArrayList<String>();
    for (itemModel obj : objects) {
        strings.add(obj.getName() + ";" + obj.getNumber() +"n");
    }
    for (String s : strings) {
        output.write(s.getBytes());
    }
fc.responseComplete();

@Override
public String toString() {
    StringBuilder builder = new StringBuilder();
    builder.append(name);
    builder.append(";");
    builder.append(number);
    builder.append("n");
    return builder.toString();
}

List<itemModel> itemModels = gson.fromJson(jsonString, collectionType);
         for (itemModel itemModel : itemModels) {
               output.write(itemModel.toString().getBytes());
        }