6 YO water heater, if it's never been flushed, could have sediment build up in it. You need to replace a leaking pressure valve. And as others have said, are you measuring 6 gals of hot water only to know? That's hard to do given that the water in a water heater is not finite. As you draw hot water it's replaced by cold water so measuring 6 gals of hot only is pretty hard. And the incoming water temp will vary from CG to CG and season to season. Heating water that's 55 degrees will be harder than 70. What may be happening is that it can't heat the water as fast as it used to. I would approach in this order - Make sure your outside shower is off, replace the leaking valve, flush it. If that does not fix it, replace the rod.
