This is the main gas control for this boiler From here on I will refer to it as the "gas valve" Take A good look at it, see the water stains, This gas valve is dangerous. Any gas valve that has been exposed to this much water MUST BE REPLACED.
Ok Now we Have Dielectric unions to repair, A bad Thermocouple To replace, A bad gas valve to replace, and this boiler is dirty. So As Always First things first. Turn Off the Power And The Gas To This Boiler.