I believe they're intended to be Zouaves/Chasseurs from the 84th New York (14th Brooklyn), the "Red-Legged Devils". Historically, I'm not sure that they would have been immediately surrounding Reynolds when he was shot, although they certainly were in the neighborhood, and a handful may have been around him. If memory serves, they were closer to the McPherson farm itself, and may have been moving toward (or engaged with) Davis's brigade in the Railroad Cut on the other side of the Chambersburg at that time.
thats the dumbest scene in the movie. The general falls there are Sharpshooters in the woods they are wearing red (they stand out) theres Confederate Artillery near by and your going to Gather around and look at a Dead General. One well placed Case shot or Canister and it would have gone from one dead man to 50 or more. The place he was shot is just below the crest of McPhersons Ridge as you near the woods closer to where Stone-Meredith Avenue intersects Reynold's Lane and is almost at the tree line where his equestrian Monument is, is on the Chambersburg Pike where Stone Meredith intersect that Road. Where Reynolds fell the Iron Brigade was posted (called the Black Hats in the Movie)
The movie scene was done to simulate a painting of the incident just like the interview between Chaimberlain's brother and the three rebel prisioners which is a famous Brady photograph of the three prisioners. A dead general especially a highly regarded one like Reynolds would have been a thing to see. Soldiers do not always behave with the most prudence as in bunching up to see the show.
I would question how many were still wearing the elaborate garb at Gettysburg or if they were wearing more stangard issue due to wear and tear and paracticality.
It is a movie after all and the fancy uniforms are much more eye catching than a blue blouse and trousers.
Yeah stuff like that in movies is called creative license.
The Unforms were very elaborate throughout the war Zouaves wore a mix of different Uniforms the 84thNY (14th Brooklyn) had what you see in the Movie, the 9th NY also Zouaves wore almost the same thing except dark blue the 114th PA (same lighter blue then the 9th) and the 146th NY after Gettyburg Zouaved and wore Light Blue. The 79th NY left the city wearing Kilts (they were one of the few that went standard Issue)
As a reenactor I have worn the 9th's Uniform for the Park Service Documantry at Antietiam and it is 10X more comfortable then the Standard Issue Uniform the most comfortable is the 79ths Dress Unifrom on a Hot day but I can't wear the Kilt in Battle.
Typically with Volunteer or Militia Regiments the States Provided Uniforms. The Standard Issue Uniform is a Federal Government thing so Units like say the 1st US Infantry would be clad in a Government Contracted company who would mass produce a uniform for the lowest price. State Uniform like the Federal would be lower quality and provided again by who ever gave the lowest bids. Typically Militia Regiments such as the 2nd Wisconsin (who wore gray at 1st Manassas which lead to tragdey and switched afterwords to standard with Hardee Hats) were Privatly funded and would chose uniforms they liked over what was Practicle. Many Units both North and South were formed by men who were wealthy and provided uniforming based on something popular the Zouave was highly popular following the Crimean War in Europe like the 1st Louisiana Special Battalion (Wheat's tigers) or 1st Louisiana Battalion (Coppens) took the Zouave Uniforms (these are French Algerian uniforms) and the 9th New York or 14th Brooklyn/ 84th New York also chose a Zouave Uniform. Much of these Uniform issues does lead to Mass Confusion mostly at 1st Manassas/Bull Run. I can Honestly say at a distance it is Impossible to identify the 14th Brooklyn from Coppens' Zouaves the tassle on the Fez is the only difference and if you can see it, it's already too late