Most mods aren't working

Probably a total n00b problem, but anyway: I just got GMod, and I have just downloaded a dozen or so mods and maps. However, only a few will work, and even then they have issues. For most of the maps, a good amount of the stuff there will either be covered with purple-and-black checkerboards (for textures, I am guessing, mostly large flat surfaces), or just be a giant floating red ERROR (guessing for props).

Also, the mods themselves. A few, such as Garry’s Bombs 3, will “mostly” work. Though most of the bombs are fine, one gives some error when it goes off and results in no explosion, and at least two have the abovementioned ERROR in place of an actual graphic, though still blow up when I trigger dynamite near them. Most of the other mods will not work at all. I got several vehicle packs, for example. One or two display in the Props menu, instead of the Vehicles one, so I cannot drive them. Some do not appear at all, unless I dig through for them manually. Also, the Portal Gun is in the Weapons catagory, though I get about a hundred errors when I attempt to fire it (I got the bad coder achievement within moments).

I have tried extracting the folder into addons, as most things say I should, as well as putting things in their places (materials, sound, scripts, ect: the folders inside the main one). Despite this, they just WILL NOT WORK. It’s getting rather annoying, not being able to use this stuff. It’s probably just a stupid mistake, but after fiddling for several hours with it I cannot figure it out. The regularly suggested general fixes (restarting steam/computer, updating graphics drivers, ect) have done nothing. Please help!

make sure wen u get something u read any files that say “read me” and make sure u put the files in the right places

The settings folder idk where to put it.