If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in Gibraltar.
It is prohibited to sell alcohol drinks to anyone under the age of 18, other than the following two exceptions. The minimum age to be served in licensed premises is 16 if: The alcohol beverage is beer, wine or cider below 15% ABV, or The alcohol beverage is served in a bottle, or a pre-packaged container below 5.5% ABV.
You can find more legal details and links here:
Wikipedia Legal Drinking Page