There's a reason why G-Max Eevee hasn't been show off yet despite being a Mascot Pokémon.
It's so obvious Eevee won't evolve
That is not a fact no matter how you try to argue it. Also it’s never once been established in the anime that a Pokémon that can Gigantamax can’t evolve.
They specifically mentioned multiple times that Eevee has different evolution/paths it could take.
Also if Eevee was truly completely incapable of evolving then it would be pointless for them to emphasize that it can evolve into different Pokémon and to come up with the idea that the reason it’s not evolving is because it’s unsure what path it takes.
Honestly given what we’ve seen of Journeys so far if one thing is clear it’s the fact is we don’t truly know what path Eevee will take and whether it will evolve or not.
Any thoughts that it’s definitely going to Gigantamax, Evolve, or not evolve is just speculation nothing more and nothing less.
Let’s not forget this is the same series where some people tried to claim that it’s so obvious that Ash won’t catch a Dragonite in episode 10, that Sobble is going to Ash while Galarian Farfetch’d is going to Goh, that Sobble isn’t evolving, that Goh isn’t going to keep Suicune.
And yet at the end of the day look what happened despite it being so obvious that those events would play out that way instead they played out the exact opposite.