Lets assume you have a class called Person with just name
private class Person {
    public String name;
    public Person(String name) {
        this.name = name;
    }
}
Code:
Gson g = new Gson();
Person person = new Person("John");
System.out.println(g.toJson(person)); // {"name":"John"}
Of course the Gson jar must be on the classpath.