Representative Matt Gaetz, Republican of Florida and a close ally of former President Donald J. Trump, is being investigated by the Justice Department over whether he had a sexual relationship with a 17-year-old and paid for her to travel with him, according to three people briefed on the matter.Investigators are examining whether Mr. Gaetz violated federal sex trafficking laws, the people said. A variety of federal statutes make it illegal to induce someone under 18 to travel over state lines to engage in sex in exchange for money or something of value. The Justice Department regularly prosecutes such cases, and offenders often receive severe sentences.