UK and EU (including key talks with Spain and Gibraltar) have
agreed a deal on border control between Gibraltar and Spain. When arriving in Gibraltar, travellers will undergo checks by both Gibraltar and Spanish officials. This will remove the need for border checks between the two countries.
As part of the deal, the government says there will be dual border control checks for arrivals by air at Gibraltar airport, carried out by Gibraltar and Spanish officials, as well as arrivals by sea at the port.
Immigration and law and order in Gibraltar will remain the exclusive responsibility of Gibraltar's authorities.
Spanish officials will be responsible for ensuring the integrity of the Schengen Area, in a model the government says is similar to French police operating in London's St Pancras station.
I've not read the detailed agreement (just the news reports), but I would assume that entering Gibraltar, one would likely need to have the necessary documentation to be able to enter Schengen as well. Not sure what that means when ETIAS gets rolled out completely. Also if there are no checks at the Spain-Gibraltar border, access to Schengen would likely mean defacto access to Gibraltar.