Well, I've seen examples where GPT-3 strayed quite amusingly. I'm hardly an NN expert, but my understanding that one can't assure the model wouldn't do this (beside retraining on the gamedata corpus alone, which would obviously impair the NN). There are good reasons for game devs to want a measure of certainty here.
Someone else in the thread suggested using a verifier based on game data and maybe that would be fine. The key IMHO must be some kind of NN trained only on game data, either GPT-3 itself or a verifier of some sort.
Usually, it's fine-tuning (training a model starting from an earlier model). They can use a small amount of text to fine-tune GPT-3 to their liking.