Gray rock means becoming as boring and unreactive as a gray rock so a manipulative person loses interest in targeting you. You keep replies brief, neutral, and free of emotion or personal detail — because your reactions and the drama are what they feed on. It's a coping tool for situations you can't yet leave, like co-parenting, not a way to repair the relationship.
Gray rock is useful for low- or no-choice contact (a co-parent, a coworker). It is not a substitute for safety. If someone escalates when they lose control, prioritize a safety plan over any technique.