[folivier]

If that is a doubler there should be a valve on the inlet side and then a quick connect on the outlet side downstream of that gauge.
The doubler has 2 pistons and works similar to an air compressor using your supply air to activate the pistons and increase the air pressure to a higher pressure that is set by a large black knob (pull the knob out to adjust). At least that is how the SMC doubler that most of us use.
It is typically used only to air up your tires since the tire pressure is close to the maximum coach air system and when airing up tires you quickly use up that higher pressure head and have to wait for the pressure to build up to max again. The doubler works at a much lower supply pressure and allows you to air up tires much quicker.