Audi 100 G-Kat Retrofitted and still control 300€ ?

Hello people, I hope I can help me a little bit further. And my grandmother asked me if you can’t do something with the “tax”. Now I’ve made a little smart and found out that a G-cat has been retrofitted. At the registration office he has been admitted to Euro 1. Now my question: Is it not even Euro 2 when a G-cat has been retrofitted? or can you do something else dami not charge 300€? Friendly greetings Nico Thank you for your answers