This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
Coffee Tiger, Wehrmacht [#savetf2], , masascuate666, gabrielangelatencio, m.popeniuk, africanlady96, chicken.., edoardovincenzi500, Noookia, alexander.fray004, Vizirus, Brazou, sfisicaro68, Smoke, marinagechtler, Mental Illness, lseduard12, twitch.tv/nossajess, kbbialy * | CASEDROP.GG, el banano, bekcibasi347, Boyovcazinandresuaqraliaro, Wizard, insert quality name here, rplekh, IamMONO, TomásCF_88, Lehr, gosth-killer123, mak9144292170, Doyle, nestoralfredo28, Wiltz, Mister RP Officiel, engineer gaming, Airon, Dim0n_, erick_chavez94, `Key-Drop.pl Tacos, Itz.Hannahhh, HANS, Radkos_CZE, киска, SonGokukid, tatyana.ivanova81.81, Sekouizie le vré, lena-konovalova91, AironHerder, gunersimal +200 others