I am interested in the origins of the name Gellert (of Gellert Grindelwald)? Has J.K. Rowling said anything about this? Is there a connection to the surname of the German writer Christian Fürchtegott Gellert (who wrote just before the Sturm und Drang period, which is connected obviously to Durmstrang, where Gellert Grindelwald was educated).
As far as I know, J.K. Rowling never has addressed the source of Gellert Grindelwald's first name. Some speculation about the name exists online, but I have found nothing definite. Even aside from Christian Fürchtegott Gellert, there have been a number of famous Europeans with the last name Gellert or Gelert, including an artist, a gymnast, and a politician. There is also a Swedish writer named Gellert Tamas, a bishop known as "Saint Gellert" from Hungary, and a "Saint Gelert" who lived in Wales. Budapest, Hungary also has a hill named "Gellert Hill." The name has different meanings in different languages: in German, "Gellert" probably derives from the word for a town crier, while in Hungarian, it is a translation of the German name "Gerard," which means "spear strength." (Sadly, I could not find a meaning for "Gelert" in Welsh, although it seems to be the Anglicized form of "Celer" or "Kellarth.")
Probably the most intriguing theory connects Gellert Grindelwald's first name to the legend of Hound Gelert from Wales. A Welsh prince called Llewelyn the Great supposedly left Gelert, a dog given to him by the king of England, to guard his baby while he went hunting. Llewelyn returned to find the bed empty, the baby missing, and the bedclothes soaked with blood. He thought that Gelert attacked the baby, so he killed Gelert. Then, looking for the baby, he found it safe, with a huge wolf lying dead nearby. Gelert had saved the baby's life, and had been rewarded with death. In the Welsh story, as in Dumbledore's life, there was a violent conflict with a dangerous creature/person, a perceived betrayal, and an innocent, much-loved creature/person that died by accident.
One of the biggest problems with this theory is that Gellert Grindelwald has a German last name, making a Welsh origin of "Gellert" less likely. But it is also possible that Rowling could have been interested in the Welsh story and simply taken advantage of the name's international nature by giving it to Grindelwald.