There is a sample from google gson documentation on how to actually convert the list to json string:
Type listType = new TypeToken<List<String>>() {}.getType();
List<String> target = new LinkedList<String>();
target.add("blah");
Gson gson = new Gson();
String json = gson.toJson(target, listType);
List<String> target2 = gson.fromJson(json, listType);
You need to set the type of list in toJson method and pass the list object to convert it to json string or vice versa.