No, I have not yet seen GMK. Yes, I know that he's a guardian of Japan. But there have been at least three different King Ghidorahs.
The first one was in Ghidorah the Three Headed Monster through Godzilla on Monster Island. Arguably, this Ghidorah was also the one defeated by Mothra in Return of Mothra 3. This Ghidorah destroyed the Venutian civilization (Martian in the English), allied with Xians, Kiilaks and giant Nebulan cockroaches, and was determined to have spent his youth killing off the dinosaurs for fun. This Ghidorah is decidedly evil.
The second one was the King Ghidorah from the Heisei-era Godzilla vs. King Ghidorah. He was created from three Dorats, was a tool of evil future Caucasians, and was killed and rebuilt as Mecha-Ghidorah. Probably neutral in DnD terms, as it shows animilian intelligence and is little more than a pawn.
The third Ghidorah was a guardian of Japan in GMK: All Out Monster Attack. There, he aided Baragon and Mothra from defeating a Godzilla created by the manifestation of Japan's war-dead. this Ghidorah is good.
The Ghidorah whose stats appear here is the first one, with one element (the electric bite) stolen from the thrid one.
I apologize for the length, but I know my kaiju.
Demiurge out.