Flights

Very limited flights due to amber status and the need to quarantine on return. More importantly, have you got your residencia sorted yet? Without a TIE or golden visa, you'll only be allowed in for 90 days, assuming you haven't already spent any time in a Schengen zone country this year. See gov.uk Spain Travel advice, and Facebook 180 days in Spain. Good luck!

Hi how can I apply for TIE 

If you weren't in Spain on brexit day 31 December I don't think you'll have a right to a TIE. You can apply for residency through the Spanish consulate in the UK before you go (you can't do it from there) and you'll need to show you have adequate income (the figure's over 20000 euros pa), somewhere to live, and private health insurance. Retiring to the beautiful island can still be done, but there are many more hoops to jump through after Brexit. Lots of information on the web, as I say.

AirEuropa have regular flights from Gatwick via Madrid to GC as do Iberia from Heathrow, again via Madrid. Also, I believe EasyJet have flights still operating to Tenerife, you can then book a flight with BinterCanarias or CanaryFly on to GC.