Does ius sanguinis mean that I am entitled to German citizenship if I have German great-great-grandparents, even if they left Germany generations ago?
Possibly, but not automatically. You are a German citizen under ius sanguinis if your ancestors had German citizenship at the time of the birth of the next generation and passed on this citizenship respectively. It is therefore necessary to find out the exact timeline of events to determine if your ancestors might have lost their German citizenship (e.g. by giving it up voluntarily, or by accepting a foreign citizenship) or if they still had it and could thus pass it on. You see that this requires a lot of research into your family history and into the respective laws of the relevant points in time. But if you are lucky, you might have German citizenship even if your parents never knew about it and neither you or them have ever been to Germany or even have a German passport.