Integrating GWT Remote services is a bit tricky within the LabKey framework.  Here's a technique that works.

1. Create a synchronous service interface in your GWT client code:

    import com.google.gwt.user.client.rpc.RemoteService;
    import com.google.gwt.user.client.rpc.SerializableException;
    public interface MyService extends RemoteService
    {
        String getSpecialString(String inputParam) throws SerializableException;
    }

2.  Create the asynchronous counterpart to your synchronous service interface.  This is also in client code:

    import com.google.gwt.user.client.rpc.AsyncCallback;
    public interface MyServiceAsync
    {
        void getSpecialString(String inputParam, AsyncCallback async);
    }

3. Implement your service within your server code:

    import org.labkey.api.gwt.server.BaseRemoteService;
    import org.labkey.api.gwt.client.util.ExceptionUtil;
    import org.labkey.api.view.ViewContext;
    import com.google.gwt.user.client.rpc.SerializableException;
    public class MyServiceImpl extends BaseRemoteService implements MyService
    {
        public MyServiceImpl(ViewContext context)
        {
            super(context);
        }
        public String getSpecialString(String inputParameter) throws SerializableException
        {
            if (inputParameter == null)
                 throw ExceptionUtil.convertToSerializable(new 
                     IllegalArgumentException("inputParameter may not be null"));
            return "Your special string was: " + inputParameter;
        }
    } 

 4. Within the server Spring controller that contains the GWT action, provide a service entry point:

    import org.labkey.api.gwt.server.BaseRemoteService;
    import org.labkey.api.action.GWTServiceAction;

    @RequiresPermission(ACL.PERM_READ)
    public class MyServiceAction extends GWTServiceAction
    {
        protected BaseRemoteService createService()
        {
            return new MyServiceImpl(getViewContext());
        }
    }

5. Within your GWT client code, retrive the service with a method like this.  Note that caching the service instance is important, since construction and configuration is expensive.

    import com.google.gwt.core.client.GWT;
    import org.labkey.api.gwt.client.util.ServiceUtil;
    private MyServiceAsync _myService;
    private MyServiceAsync getService()
    {
        if (_testService == null)
        {
            _testService = (MyServiceAsync) GWT.create(MyService.class);
            ServiceUtil.configureEndpoint(_testService, "myService");
        }
        return _testService;
    }

6. Finally, call your service from within your client code:

    public void myClientMethod()
    {
        getService().getSpecialString("this is my input string", new AsyncCallback()
        {
            public void onFailure(Throwable throwable)
            {
                // handle failure here
            }
            public void onSuccess(Object object)
            {
                String returnValue = (String) object;
                // returnValue now contains the string returned from the server.
            }
        });
    }