This download looks like an unpacked addon. You can either convert the files to an addon or merge the folders in the zip with the folders in your Garry's Mod folder.
You should end up with a directory structure similar to below:
This option makes it difficult to remove the content should you no longer want it. Merge any folders from the zip with the equivalent folders in your Garry's Mod folder.
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
fafarobert, 게못하는 게임백수, Last Standing Poro, colbypopkie, galusins56, HelloMikey, Yung Mind, DICK, BloodierWrex, kazuo, yudecchi, Loyf, naneth kyuren, naneth kyuren, MarineMechX2, last(p0008874), kdragon410, kdragon410, Schranz, [CN]Izayoi Sakuya, Ramgier ヽ(｡･ω･｡)ﾉ, El Demolitor Cool, mushsky, chlrud951, Big Sipper, MrWARIO (dead), 日野原 真, Theeddstar93, BigWAT, burningice27, Flanster, ルーミア☆Fortress 2, gh2501, GrouchyGeneralReimu☯☣, 할짓없는놈(Mr.Halt), Echo, gea.christopherferreira, HATCH_END, fedormchugh, >>Great White<<, >>Great White<<, lonely19682005, sparta567890, Xyno, Xyno, The Last Living Spycrab (ze), Jackster743, Sokkah Abdelkader