This is part of the Environmental Control System (ECS), made by Airesearch Manufacturing Company for the Gemini program. It created a safe and comfortable gaseous atmosphere for the astronauts, providing fresh oxygen, pressurization, temperature control, water removal and toxic gas removal. The ECS had four systems which essentially operated independently of one another - oxygen supply system, cabin loop, suit loop, water management system. This ECS was not flown in space but used for testing. NASA transferred it to NASM in 1969.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.