Extract the contents of the zip into your main Gettysburg folder.
I've modified the tree graphics to suit my taste. The originals were too bright green for my liking. I don't claim my trees are correct or more realistic I just like how they look better.

I used Paint.NET for this. The exact change I made was under "Hue & Saturation"
I applied -5 Hue, -10 Saturation, and -30 Brightness.
Modded Trees:

Vanilla Trees:

Click the images for full size.