In a move to simplify movement at the border between Spain and Gibraltar,the EU have asked the British government to agree to removing the border fence that now separates the areas although there is freedom of movement across the border. It’s a simple but complicated situation.
While the UK was a member of the EU, the issue was dormant because of agreements on freedom of movement within the Union. Brexit changed that, and to avoid a new ‘hard border’ between Gibraltar (non-EU) and Spain (EU), the parties agreed that Gibraltar would join the Schengen zone, which Spain was already a member of, thus ensuring freedom of movement.
At present, therefore, the 15,000 or so Spanish workers who cross each day to work at jobs in Gibraltar, can move freely, just showing their national ID card, as can Brits and others who are Gibraltar residents. However, UK nationals living in the UK must show passports to enter the Schengen zone, and will soon need ETIAS permits as well.
The Spanish proposal would remove the physical fence, and give Spanish authorities a monitoring role over entry from outside Schengen. That would include a role at Gibraltar’s airport and seaport. The UK has not yet responded.