Since your Java class doesn't resemble your JSON in any way, shape or form ... you're going to have a problem with that.

• Problem #1: OperationResult should be an int
• Problem #2: You're declared ResultData as an Object ... Java doesn't work like that.

You need your POJO to match the JSON:

public class ReturnData {
    public ReturnData() {
        OperationResult = Result.Failed;
        Messages = "An Error Occured";
        UpdateAvailable = "0";
        ResultData = "";
    }

    public int OperationResult;
    public String Messages;
    public String UpdateAvailable;
    public MyResultData ResultData;
}

class MyResultData {
    public String SessionId;
    public String UserName;
    public String AccountId;
    public List<String> Roles;
    public String DisplayName;
    public int Status;
    public int Type;
}

ReturnData rd = new Gson().fromJson(jsonString, ReturnData.class);

I'd also consider using Gson's @SerializedName("name") annotation to convert the PascalCase field names in your JSON to camelCase field names in Java.

@SerializedName("OperationResult") public int operationResult;