There are two stories -- one is that he was killed by grapeshot, and the other told by his aide, Captain John S Jones who was with Garnett at the time. Jones stated that he (Garnett) was killed instantly by a musket ball through his brain. This was reported in the Pennsylvania Daily Telegraph 7 July 1863.
Further, Colonel Norman Hall, whose brigade assisted that of Union General WEbb in beating back the Confederate attack, reported that general Armistead and Garnett were picked up near the point of the Confederate breakthrough at the stone wall, along with colonels and officers of lesser rank. Colonel Hall's statement was made within two weeks of the attack.
Would greatly appreciate any comments -- hope there are others who would check out these reports.
thanks.