It is obvious that there are serious differences of opinion in the Spanish front over what should be installed at the land frontier which would be against Spanish interests. Right-wing Spanish sources, and now elements in the security forces, are diametrically opposed to other Spanish involved in the current negotiations who are taking a softer line on Gibraltar.
They see Spain as losing an opportunity to claim joint sovereignty, and demand that Spain's National Police and Guardia Civil should continue to exert controls at the land frontier as at present, as such access point is linked to Spanish territory.
The UK/Gibraltar position, according to the right-wingers, is that to allow the Spanish National Police and Civil Guards control the land frontier would constitute 'an attack on the sovereignty of the United Kingdom.'
Such Spanish sources say that the initial position of the EU is for Spain to control airport and port frontier, but Frontex being given the task at the land frontier which is adjacent to Spanish sovereign territory.
But Spanish security sources are quoted as saying that Frontex controls at the frontier would be a cession of Spanish sovereignty. And after the initial 4-year spell, the UK position would streng-then itself and it would be more difficult to change it.
Of course, whatever position is taken by Spanish negotiators, the official Spanish position is that Gibraltar destroys Spain's national unity and the territorial integrity of Spain.
To add to their arguments, right-wing Spain bring up that Gibraltar is a fiscal paradise and some act in irregular fashion, even saying that what is planned would provide the UK access into Spain via Gibraltar for financial services, goods etc.

09-11-21 PANORAMAdailyGIBRALTAR