GRA Retrofit: Impossible?

Hello.
My customer purchased a VW Golf, model year 2011, with a 66 kW 1.6L Common Rail TDI engine from a VW dealer, with the assurance that the Gasoline Particulate Filter (GPF) could be retrofitted without any problems.
When he wanted to have this done, the workshop refused, saying that it was no longer possible due to the change from Bosch to Siemens components.
I can't believe it.
Does anyone know anything about that?

best regards
Aviation diesel.

I've now found the MKB code: CAY.

Please post the complete vehicle scan.