The phrase "at the gun" means to take action at a specific time or on a specific signal, often in sports or competitions. There are several synonyms for this phrase, including "on your mark," "get set," or "ready." These terms are commonly used in racing events like track and field, swimming, and cycling. In some games like football or rugby, the referee blows a whistle to signal the start of play. The phrases "on your whistle" and "on your call" can be used as synonyms for "at the gun" in these contexts. Regardless of the terminology used, these phrases signify the beginning of a challenge or competition.