Longstreet's plan at Gettysburg was NOT correct with Lee's limited supplys he could not fall back in enemy territory, dig in, a let the enemy attack him. The enemy does not have to attack you if he can simply set and wait you out. War is as much if not more so about playing a game of chess as it is about fighting of the battles. In that situation that Longstreet proposed Gen. Meade have had all the advantages and would not have had to attack Lee. Meade's job was to keep Lee away from Washington and had all the advantages of supplies and reenforcements. With Lee's Army dug in a defensive position he would not have been threatening Washington. Meade would have only to cut Lee off from his supply lines and watch the ANV and Lee wither at the end of the vine.
And there again was one of Longstreets failings. Because he did not believe in Lee's plan on July 2nd and on July 3rd he did not oversee the planning and preparation that needed to go into either of those battles and did not execute them as they were supposed to.
His failing to supervise the placement of his troops on July 2nd caused the Confederates to lose the advantage that they had gained by overlapping the Federal left at Devils Den by losing the use of Laws Brigade, which misdirecting that Brigades attack against Little Round Top rather than quickly eliminating the federal position at Devil Den. Had Laws Brigade rolled up the flanks of the Union position at Devils Den and advanced quickly up the valley behind that position rather against Little Round Top the entire Union left would have collaped and one brigade of Unio troops there on Little Round top would have had to be abandoned their position or they would have been isolated.
On July 3rd His delay in committing Pickets men forward until the Artillery Ammunition was almost expended was a serious mistake that meant the infantry had to advance without the Artillery support it needed.
Longstreet was a good 2nd in command General under the steady hand and the leadership of Gen. Lee. But he sometimes acted like a spoiled child when he didn't get his way and in my opinion that is why he didn't get his independent command as he wanted.