If the Confederacy had won at the Battle of Gettysburg

[True Federalist (진정한 연방 주의자)]

Say the Confederates win the third day of the battle. Why does the war end?

The best scenario for this to happen is if Meade is forced to Surrender. If Meade surrenders then Lee's Army can take Washington, Baltimore and even Philly because their isn't a strong enough contingent of troops that can resist the whole of Lee's army.

How exactly is this supposed fantasy of a surrender by Meade going to happen?.

Lee is in no condition to besiege Meade and the Civil War was one in which the defense had superiority. At no point was Lee ever close to crushing the Army of the Potomac. While it is conceivable for Lee to have forced Meade to temporarily leave the vicinity of Gettysburg, he's still left with no viable option other than to do as he did after Sharpsburg and return home.

Moreover the defenses around Washington garrisoned by the XXII Corps were sufficient to keep Lee out of the Capital. Lincoln may have worried overmuch about the defense of Washington City, but not without reason. While the XXII Corps certainly could not have taken the field against Lee, they didn't have to. They only had to hold their own and were sufficient to that task.

Lee did about as well as could be expected in his 1863 campaign. One way he could have done better would've been if the Army of the Potomac had dashed itself to pieces against a dug-in Army of Northern Virginia which I can't see a survivor of Fredericksburg such as Meade, or even Hooker, attempting.

The other would have been for Lee to keep the Army of Northern Virginia largely intact and conceded Gettysburg after the second day.

[True Federalist (진정한 연방 주의자)]

The South would've won had it successfully seized enough Northern supplies. Had Maryland and Delaware seceded, the Northern wargoal would have shifted to controlling the border states, and had the South invaded and conquered Philadelphia from Gettysburg, the war could end early. The impact of the lack of the Emancipation Proclamation cannot be understated as a factor either; if Lincoln hadn't issued this, then at least one or two Reconstruction Amendments would have never occurred, if not all three.

The Emancipation Proclamation was the year before, just after Sharpsburg, not Gettysburg. The supply problems that the Confederacy had were more about transport than materiel, and much more about keeping the civilians supplied than the army.  The South was not and never had been a Jeffersonian fantasy land of independent yeoman farmers. The regions where it came closest to that "ideal" were some of the poorest parts of the South and the parts with the strongest Union sentiment during the war.  Lee's summer campaigns of 1862 and 1863 were much more about keeping the Union Army too busy to go rampaging through the Confederate hinterland than gaining supplies for the Confederate hinterland.  Lee tried to do the same in 1864, but didn't have the resources to pull it off.

By 1863, the Confederacy had no chance of winning the war, tho the Union could still have lost it.