“The reason the Gr’a argues on the Ramba’m”
He doesn’t just argue on the Rambam, he argues on the Shulchan Aruch and the Rema as well.
My point was twofold:
Rishonim are not Chaza’l. The concept of ??? ?’ ?????? was never aplied to Rishonim, AFAIK. There was a chasimas hashas, at which point later authorities will not overrule earlier ones, they will only interpret and choose an opinion to follow from those offered by the Amoraim. After that, for example from the works of the Rishonim, one can argue.
Nevertheless, it’s uncommon to find an Acharon disputing a Rishon, because it was generally accepted that the Rishonim were superior.
In one particular case, Y’D 179:6, the Gr’a takes issue with the Rambam’s rationalist approach, stating that it was influenced by philosophy, and that his approach was already rejected by many after him.
I wasn’t addressing the fact that there are halachic disagreements between the Vilna Gaon and the Mechaber or R’ma.