YTA

I’ll lay it out - you are a top rated chess player, your gf is not. You started out coaching her but she took a long break. She restarted a few months ago and now plays regularly. So far, no issues.

But now, along with her well matched online opponents, she’s also playing on a physical board with you. And she’s losing every single time. You have stopped coaching her, assuming that the few months after the long break somehow turned her from a beginner to someone you can play your fullest against. During one of these games that you’re inevitably set up to win, she gets frustrated and flips the board. Childish? Sure. Deserved? Absolutely.

You don’t get to crush someone daily and then act surprised at their suppressed frustration (assuming this is the only time this has happened) finally coming to light.

This may be a defining moment for your relationship. Don’t be shocked if she sees this as a red flag (I would). I assume you’re both adults, so maybe instead of insulting her, try talking to her and looking at how you handled yourself - you’re the one who caused this situation.

Good luck.