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:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
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![[ACF-3] GFV Mk. 1 Medium Tank](https://filecache.garrysmods.org/62189/1/570x180.jpg)