My buddy has a 2009 era (aluminum boiler?) Gaggia Classic. he ordered the 73x57x8.5 mm gasket (2) from WLL but asked me to help when the old gasket was jammed. So together we replaced the gasket. And I ran blind portafilter group clean cycle. He took it home to discover lots of leaking so I got the machine back. The new gasket had split along the circumference in the center of the thickness of the gasket about 2 cm (WTF!? as they say) so leaking occurred. I changed to the second new gasket tested it successfully and returned the machine to him and he tells me it still leaks around the portafilter.

I can't see any problem with how I installed it.

Does this machine need a 73 x 57 x 9 mm gasket?

Is there some trick to seating the gasket correctly that I don't know such as use of shims?

I've gotten recommendations for this one from the Gaggia Users Group
https://www.amazon.com/gp/product/B00OXA72RW/

I have the feeling he is not tightening the portafilter enough with the new gasket. When new you get resistance soon and tend to stop tightening, tell him to get it as close to perpendicular or tighten til it stops leaking. After about a week it will then easily slide to a non leak position from break in.

My experience after owing one since 1991.

Chert wrote: Is there some trick to seating the gasket correctly that I don't know such as use of shims?

The stainless steel boiler Classic has shims in the brew group. The old and much better (!!!!) aluminum and brass boiler Classic does not.
Forget you ever heard the word 'shims'....

My friend brought me the machine and I demonstrated that with a tight position no leaking was occurring with two ristretto shots I pulled. He took it home and reports that it leaks for him as bad as ever. I wonder if I may have damaged new gasket with the firm seating of tightening it on beyond the perpendicular to the face of the machine.

So... once more he brought the machine in. The rubber gasket was split in two place from my overzealous tightening of the portafilter handle.

I installed a Cafelat gasket which engaged at about 7 o'clock. No leaks no drips. yes ristretto yes delicious.

My friend is happy again with his machine working well.