EU GDPR is a general regulation that applies all across Europe. However, EU Member States German Data Protection law has implemented the EU GDPR in some specific fields, like employment, health data, children data, consumer protection, etc. You need to consider the purpose of processing and how you will process those data. Are you asking for information about a particular category of personal data? I.e., is the survey on political opinions or sexual orientation?
You need also to consider if the data collected are anonymized, if they are transferred and if there is any personal profiling or automatic decision.
All these elements will have an impact on how the GDPR will apply to the online survey.
Here you can find more information about German data protection law, GDPR, and the data processing