The part with the product ID "RA99905022" from Rocket Espresso Milano is not compatible with the Gaggia Classic. It is specially designed as an accessory for portafilter machines from Rocket Espresso, in particular for their own models and not for Gaggia machines.

This accessory is designed for use with a portafilter and can be used for 1 or 2 espresso shots depending on the configuration, but it will not fit on a Gaggia Classic machine.