Santa's data collection has always been compliant with GDPR, so he has no need to change his ways. The nature of his data collection is more transparent than most companies, and he is open to updating his records if you contact one of his representatives.
For example, he makes it clear that he is operating in your town:
You better watch out
You better not cry
Better not pout
I'm telling you why
Santa Claus is coming to town
The legitimate business purpose of his data collection is to create a list of those who are naughty and nice this year:
He's making a list
And checking it twice;
Gonna find out Who's naughty and nice
Santa Claus is coming to town
He even gives some examples of what data he's collecting:
He sees you when you're sleeping
He knows when you're awake
He knows if you've been bad or good
So be good for goodness sake!
The GDPR has some other requirements to it, such as an EU-based representative being necessary for operating in the EU, allowing users to request data updates, and getting consent for data collected.
Thankfully for Santa, he's been operating compliant representative systems for decades: Just go to any mall during the holiday season to meet with a representative. To ensure open and accurate records, the representative will ask the child if they've been naughty or nice that year and what type of present they want.
As long as the child's parent/legal guardian is nearby to confirm the data change requests, Santa will be happy to update his database to ensure the naughty/nice data is accurate and that the requested presents are delivered.
As for consent, the children are obviously too young to provide consent and must rely on their parent/guardian to consent to Santa's data collection. I doubt there's a single house that is receiving presents from Santa without the parent's explicit consent, and I'm sure we've all been told by our parents at some point to "be good or Santa won't give you presents this year!".
