UK flight to Gibraltar then entry into Spain ?

Of course you can. Just bring your Within-72-hours-Covid-free-papers with you. I hope your Spanish is good enough to convince any  suspecting Immigration police or ordinary police of your aims.

As I don't know your reasons for travelling, I'm not going comment on the rights and wrongs of travel, bu given you two posts I guess you are looking at how best to work with the current restrictions.

Whichever way you choose  to travel, the only advice I can give you is to have as much paperwork to hand to justify your journey as you can, and be prepared to be denied passage at any nationa,  local or regional border if you are unable to satisfy the questioning authority as to your reasons for travel.

No frontier restrictions in place in either direction - 785/2020

NOVEMBER 10, 2020

Her Majesty's Government of Gibraltar can confirm that crossing from Gibraltar into Spain continues to happen and flow normally. 

The Government of Gibraltar has not applied any restrictions at the frontier on entry into Gibraltar. 

The controls on movement in Spain imposed by the national and regional governments in Spain does NOT affect the right of access to Spain from Gibraltar. 

Once in Spain, the relevant restrictions apply.  Citizens should familiarise themselves with these restrictions.  In short, the Government understands that movement outside of La Linea and back will only be possible for certain limited purposes (such as work, medical appointments and other essential circumstances).

The Government will continue to make information available as and when circumstances change.

The above from the Gov's website. Seem the real problem / restriction is travelling out of La Linea and into & through the other municipaities

As Beannie says, you risk being stopped and turned back at the various municipal borders you will cross.

On top of that, your country of origin will be questioned and once it’s ascertained you slipped through a loophole and came from UK and entered Spain without a PCR test, it’s entirely possible you’ll be obliged to take one at your own cost and subsequently heavily fined (up to 6.000€ I think it is now)

You may only travel now if you are going to a primary residence and for that you would need to have your Residencia card /TIE

Spanish citizens have the right to travel back to their own country, as do U.K. citizens in the other direction. In addition if you have residency/TIE here in Spain you also have the right to travel to your home. People I know have recently hired a car from Gibraltar, to visit family living here in Almeria. They were stopped on the Motorway at a roadblock at Nerja but once they showed passport, plus driving licence and boarding tickets from Gibraltar and said exactly where they were headed, they were allowed to continue.

Chris