I had a look on findagrave.com, but wasn't able to find your great-grandfathers grave, however, not all headstones are included, so that doesn't mean much.
It seems that there is only one cemetery in Luckau, so you could try contacting the city to see whether they have an index of who is buried where. The page here includes an e-mail address you can try: [email protected] (though note that is is generic "city" address).
GoogleTranslate can help with writing an email to the city, alternatively, people on the Genealogy Discord or the WikiTree Translation Team can help with translation.