The General Data Protection Regulation is a law which protects the data and privacy from all EU-citizens.
The answers which your users provide to LikeKnob could be seen as personal data from regulators, so the GDPR also applies to LikeKnob.
The data is store within a european (German) Data Center, so you don’t have to worry about Privacy Shield and data being exported out of the European Union.
IMPORTANT: I am not a lawyer. This is not legal advice.
I am not 100% sure if the data collected by LikeKnob is even considered personal data, but I will advise you to be open with your users and visitors.
Let your users know what data is saved about them in a simple understandable manner.
LikeKnob Rating Bar and Answer Field If you rate a page via the rating bar and/or the answer field, your answer, rating, current date, and the current url will be saved by LikeKnob. This data is not merged with other data sources. Your current rating and answer will be saved on your local computer to prevent you from rating a page again. It is not used to identify you. This data is processed on the basis of your consent (Art. 6 para. 1 lit. a GDPR). More info on their website