With this mod you'll change the in-game Gallimimus to a more accurate one; with this mod, however, I had several problems with the rig, meaning I wasn't able to give the animal the accurate look I wanted.
What was impossible to change into its more accurate form was based on the JP Gallimimus, which is the most accurate in the "Jurassic" franchise.

A full list of the changes:
-Remade the cranium to be longer and slender, reshaping the Nasal, Premaxilla, Maxilla and Lacrimal
-reshaped the jaw accordingly to fit the cranium
-deleted the teeth (texture still shows them for some reason, will be fixed as soon as I learn how to skin)
-reshaped the neck to have better volume and a slightly more accurate posture (changing length breaks the animations)
-removed the hump above the shoulders
-evidenced the muscles along the body
-gave the animal a gastralia and coracoid bone
-slimmed down the hindlimbs to make them more balanced to the body
-remade the tail in posture, thickness and length

Before installing ALWAYS REMEMBER TO BACKUP THE ORIGINAL FILES!

To install:
-go into your game directory
-copy the files into "Jurassic World Evolution\Win64\ovldata\Content0\Dinosaurs\Gallimimus"
-overwrite all
-enjoy!

To uninstall:
simply copy and paste in the same directory the original files :)