His relationship with Rocket indicates he’s a Nine, but let’s look at him in more detail.
Even though he’s a monosyllabic tree, he definitely fits the definition of Body Type. He’s quite competent as a superhero fighter, growing limbs and using his strength. He’s so cheerful about knocking people about with his extra-long arms.
He and Rocket are a symbiotic duo. Groot doesn’t have to be smart; Rocket is the brain. Rocket doesn’t have to be nice; Groot is the diplomat. In real life I’d want to break these two apart so each could grow the weaker side of their personality. Since they’re comic book characters, though, they are wonderful together. They literally speak a language only known to them.
Without this relationship, would I be able to identify Groot’s Enneagram? I doubt it. That they travel together is key. Their interaction is beautifully written and conceived.