No, you cannot use a gallon-sized bag for your liquids in carry-on luggage; you are strictly limited to a single quart-sized, clear plastic bag per passenger. This is part of the "3-1-1" rule enforced by the TSA and international aviation authorities: you may carry liquids, aerosols, and gels in containers of 3.4 ounces (100ml) or less, and all those containers must fit comfortably inside one quart-sized bag. A gallon bag is significantly larger than a quart bag and will likely be flagged at the security checkpoint, forcing you to either discard the excess items or move them to checked baggage. While there are exceptions for medically necessary liquids, baby formula, or breast milk—which can be carried in larger quantities and do not need to be in the quart bag—standard toiletries like shampoo, toothpaste, and lotion must strictly adhere to the smaller bag limit to ensure a smooth screening process.