This is a blood mod it makes real bullet holes and it looks soo much better than the origional blood you need a good video card to make the bullet holes appear but there is a mod called the better visuals mod what can fix that.... extract to /gmod/materials/decals (I did not make this so i deserve no credit for it i am just releasing it)
This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
doom3blood <-- Addon folder!
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