Noob needs help!

[ApacheShush]

ok so i am very new to gmod?garrys mod an i am having alot of trouble with alot of the maps. Both for prop hunt and ttt. IN my sandbox option i have two maps with their display pictures and the others for prophunt and ttt dont show up and are blacked out. Also in game i see alot or error symbols and purple and black blocks for walls or as the floor etc. please can someone help me for i am very new to this and would just like to enjoy the game. And yes i have read the pinned disscusions aswell.