There are several synonyms for the phrase "gat out of line." One alternative is "misbehave," which means to behave in an unacceptable or inappropriate manner. Another word that can be used is "rebel," which implies a disregard for authority or conventional behavior. Additionally, "act up" is a common synonym that suggests disruptive or unruly behavior. "Deviate" can also be used to describe behavior that falls outside of acceptable norms or expectations. Other possible synonyms could include "stray," "misconduct," "transgress," or "break the rules." Regardless of the specific wording chosen, these synonyms all convey a sense of behavior that is unacceptable or outside of acceptable boundaries.