Disclaimer: I have no law degree and everything in this post is speculative.
After reading up on GDPR (https://en.wikipedia.org/wiki/General_Data_Protection_Regulation) it deals with the transfer of personal data to entities outside the EU or EEA for processing. The definition of personal data would be the main point to see if/how GDPR is applicable to lemmy instances. (https://en.wikipedia.org/wiki/Personal_data)
Your IP address and EMail address could be classified as personal data from my point of view. But this won’t be shared or processed outside of the instance as far as I can tell. If your username and associated posts are classified as personal data I can’t say, but there seems no connection of these to your IP or Mail outside the instance. According to this TechDispatch (https://edps.europa.eu/data-protection/our-work/publications/techdispatch/2022-07-26-techdispatch-12022-federated-social-media-platforms_en) the instances still must adhere to GPDR, but as there is not much or no processing of personal data taking place this should pose no issue.
All of this is based on a bit of research, so please enlighten me if I made any mistakes.
The reactions you are seeing are based off of Metas history. We will see how it works out.
Don’t forget about the time wasted on social media or other useless stuff on the internet.