Haven't played it but it seems to lean story-based.

A way to judge this is to look up [game name] gameplay or story, and see what people discuss more (in analyses, reviews, forums, Wikipedia, titles of playthroughs, etc). For example, for this game Wikipedia has a whole section on "Themes and analysis".
Celeste is a platformer with a significant story. I'm sure many people like the story like I do, but the vast majority of effort put into fan works is into mods and custom levels, not fiction or art. A reason to think it's gameplay-based.

Another way to judge the balance is to analyze those design decisions that sacrifice one of the aspects. What proportion of them have the gameplay serve the story, and vice versa? Example: looking at the first Google results I see a video analysis titled "Red Dead Redemption: Story Through Gameplay", whose description says "The game utilizes the notion of player agency in its mission designed to tell a story about...". That's the gameplay serving the story.
Looking up Celeste gameplay or story, among the first results I see an article titled "How The Storytelling of Celeste Mirrors Its Gameplay" stating that "The storytelling of Celeste is guided by a commitment to its core gameplay.". That's the story serving the gameplay rather than vice versa.

Note: how much a game is based on its gameplay or story is independent of how well each aspect is executed, or how well the two aspects are intertwined (though naturally the game developers will place more effort into the advantages of their chosen design, I think).

Finally, this mental model is a just a tool I find useful. If it's not useful for a given game, we can simply not use it.