using Gson library in GWT client code

Gson uses Java features that are not supported in GWT such as reflection. Thus it is not possible to use Gson in GWT client side code.

Not exactly what you wrote but I guess that what you meant was how to serialize/deserialize JSON in GWT code?

In GWT 2.1.1 you can use GWT AutoBean framework

See there at the bottom of the article it has this magic code ...

String serializeToJson(Person person) 
{
    // Retrieve the AutoBean controller
    AutoBean<Person> bean = AutoBeanUtils.getAutoBean(person);
    return AutoBeanCodex.encode(bean).getPayload();
}

Person deserializeFromJson(String json) 
{     
    AutoBean<Person> bean = AutoBeanCodex.decode(myFactory, Person.class, json);     
    return bean.as();   
} 

the serializeToJson() woks fine for me even with instances that are inherit Person but I did not try the deserializeFromJson...