Was Gunsmoke still being filmed when Milburn Stone passed away?

No, "Gunsmoke" was no longer being filmed when Milburn Stone passed away. He died on June 12, 1980, and the series had ended its original run on March 31, 1975. However, "Gunsmoke" did have a series of TV movies that aired after the show concluded, but those were not in production at the time of his death.