Don't know anything about other Cemetery listings but if your gggrandfather remained in a hospital on the Champion Hill battlefield and died there it is possible that he is still in the ground there. The Union army maintained quite a large hospital on the battlefield for around a month after the battle and cared for and buried both Union and Confederate soldiers. Many Union soldiers, whose graves were marked, were disinterred in 1866 and moved to Vicksburg. Confederates buried in mass remained. I know that is no help to your search. Just giving possibilities...