EU directives such as GDPR come into power by being localised and enacted, ie made into UK law. The data protection act 2018 will cover us up until it’s reversed, which is what some of us fear.
The UK's position is strange because of Brexit, but it seems the practical position of all recent governments has been that existing EU laws should be transposed into UK national laws where necessary and with the required changes to make sense, until such time as our national authorities might decide to deviate from them. Primary legislation has already been made to that effect. The big question is what "that effect" actually is, given the huge volume of legislation affected and the potential ambiguity in what changes are required for any given law of EU origin to continue to make sense in a post-Brexit UK.