Yes, you can give Gravol to your 12-year-old daughter if she has gastro. It is available in different forms like liquid, suppositories, and chewable tablets without a prescription. Encourage her to drink small amounts of fluids at a time to prevent dehydration and try light foods like soups, toast with butter, eggs, and crackers. Avoid heavy or sugary foods like red meat and undiluted fruit juices. It is also recommended to practice frequent hand washing for other household members and disinfect surfaces and handles.