The Gibraltar government has issued advice to people who live in Spain but drive a Gibraltar-plated car or motorcycle, following a crackdown by the Spanish authorities. Over 20 vehicles are believed to have been impounded so far.
This is not related to Brexit, but is to do with customs procedures under EU rules, regarding importation of vehicles and residency status of their owner. Nobody who is officially resident in Spain can own a Gibraltar-registered vehicle, and vice versa. Even if someone has an address in both places, they cannot be officially resident in Gibraltar and Spain at the same time. Anyone in this situation who decides to make Gibraltar their official residence must legally cancel their residence status in Spain, otherwise the Spanish system will still treat them as if they are living there.
After taking the matter up with the Spanish authorities, the government advice is for those in this situation to familiarise themselves with the legal requirements in Spain and ensure that they comply with Spanish and Gibraltarian residency rules, with the help of a lawyer or tax adviser if necessary. Failure to comply wih the requirements could result in their Gibraltar-registered vehicle being impounded.