My Husband has been offered a job in Gibraltar which he is keen to take as we have been trying to make a move to Spain for years and finally the possibility is there!
However, what happens if he works in Gib and we live in Spain with regards to residency / tax? Do we still have the 6 months to apply? I would also assume that there is a tax treaty and so tax is paid where it is earned?

The tax is payed where you work, that’s correct. However you’ll need to submit tax return form bout in Gibraltar and Spain. Depending on your situation you may also need to pay tax difference in Spain. That mostly depend on allowances, some people have to pay nothing.

It’s straight forward to get Spanish residency for EU citizens. You can apply in your local police office.