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.
Big Boss, mamiderodrigo, caindude1, easy, Source SDK TV, Halo Pirate 11, scarpion312, 死肥宅, dakotapro2015, legal.hack, sew63, artwoord11, rongta, ArmyLegoMan97, Hellhound 0-5, L130R, LightFoot, SwoopDaKing, andiew2548, ★Cody D. Buni★, mateusdemelonaves, gimiiii2701, gimiiii2701, [BN] McLovin, TigerMw, Franco M, pulsdepoes, brandoneatonshr4, johaneliasen, robbed by a chicken, robbed by a chicken, OneEpicNova, ♤Isaac Clarke, ♤Isaac Clarke, omarzaneyoussef, Trevor Phillips, christianswed682, Grozger Zasrogtiveslin, chronoser163, comrade_mehmet, The Milk Man, szathmariadam28, szathmariadam28, BennyMan, ✪[SPQR] @rtem-predator✪, Fregan Blayr, ๖ۣۜΚrØnΕκ†™, /OneAboveAll\ Onurlord242, BlackAfricanTaco, YareYareDaze +20 others