Because you will not be using consent as your legal basis for processing data under GDPR, in most cases you will NOT need to re-consent existing participants (or parents/ representatives for paediatric studies) in order to comply with GDPR. Unless you are making changes to your study processes or arrangements (eg changing what data you collect or how you will hold it), you will not need to re-consent existing participants for common law or other ethical reasons.
It is important that you are clear that this does not affect the ethical importance of consent. You will also still need to obtain consent for access to or use of confidential patient information to meet the common law duty of confidentiality. There are, of course, also other requirements in law for obtaining consent for research, eg clinical trials legislation, as well as ethical reasons.