I think what you are referring to is the rose holder assembly* according to the gaggia parts diagram. Also known as a dispersion plate. It should clean up ok with a wire brush or sandpaper.
The aluminium boiler is the thing most likely to corrode, but can be replaced relatively cheaply. As long as the boiler isnt actually leaking, you can probably clean it up. A dremel or similar works well for this.
Loads of threads here and elsewhere on repairing/restoring gaggias
Oh, my gaggia was made in 1994, so yours is a youngster