We have done two major deployments of OtterTune in Europe. One of them we published a VLDB paper about (https://ottertune.com/blog/vldb-autonomous-database-tuning-i...). Their infosec people looked at the data OtterTune collects and determined that there are no GDPR issues.
We are incrementally expanding the AZs we support in the free tier in order to carefully scale our service. We plan to support EU zones by the end of the year.