air gap = separación, generally small, like between the walls of double walled pipes and hoses.

seal = sellado, sellador, junta, zapatilla

attachment = dispositivo, accesorio

normally in Spanish we do not add *separación* so I would say it is something like:
dispositivo o accesorio sellador o de sellado

but honestly it could be a ring seal, a gasket, etc. so without context is hard to say