Knowing when the wounding occurred makes all the difference. On 1 July Surgeon Legrand James Wilson (42 MS) established his field hospital in some trees near a brook (likely Willoughby Run). The most serious cases were sent further back to the brigade hospital at the Lohr farm on the Chambersburg Pike, which was run by Surgeon Benjamin F. Ward (11 MS). On 3 July, prior to the charge, those knocked out of action by artillery fire on Seminary Ridge went taken back to the Emanuel Pitzer farm directly behind the lines. Those who participated in the charge and were shot down and captured in front of the Union lines were typically taken to the various Union Second Corps hospitals on the Jacob Schwartz and George Bushman farms. Of those remaining behind Confederate lines when the retreat began, those who were physically able to travel returned to Virginia in the wagon trains, but if they worsened and could travel no farther they were left in hospitals along the route, such as at Hagerstown or Frederick, Maryland. Following the battle, (Assistant?) Surgeon Willliam G. McCreight (42 MS) was left behind to tend to the wounded who were then prisoners of war. The various hospitals around Gettysburg were eventually closed and consolidated at Camp Letterman. When recovered sufficiently they were sent on to other northern hospitals like the one in Chester, Pennsylvania. Unfortunately, aside from all the above there were a few killed who were buried near where they fell. After the war these were gathered and usually reburied at Hollywood Cemetery in Richmond. But a very few escaped even this attention, such as three members of the 42 MS who were hastily buried near the McPherson buildings near the Chambersburg Pike - their combined headboard was not found until the 1880s. Just a quick survey that I ran shows where some of the dead and wounded of the 42nd Mississippi wound up:

Samuel Lohr farm - 3
James Will's farm (not sure of where this one is) - 1
Jacob Schwartz and George Bushman farms - 7
Emanuel Pitzer farm - 2
Camp Letterman - 5
Chester, Pennsylvania - 2
Frederick, Maryland - 1
Hagerstown, Maryland - 1
Other locations (as noted above) - 3
House in Gettysburg - 1 (the Colonel)