The message is basically saying that the member logging the message is having trouble communicating with another member. In this case it has not managed to communicate with the specified member for 6.5 seconds. The typical cause of this is a big GC pause on the other member or sometimes a network issue.
Now given what you have said about your application I would think that this was indeed caused by big GC pause. Do you have GC logging for the gateway node you describe? Is it doig big GC pauses? At the end of the day a 6.5 second GC may not be too bad if your application can put up with it. It will mean that the cluster is not performing at its best though and maybe you should do some GC tuning.