to install download lml and make a stream folder in modloader>lml then drag everything in the modloader folder to your game directory.
you only need to choose one of the files unless you want to make every color of that horse breed the gang verison.

if you would like to replace a gang horse with a different breed of horse then the kind the gang horse is you can.
it doesn't matter what they are replaced with, they replace properly as least in my testing. 
