Bold text means it's a folder, non-bold are files
If the folder structure is like this
ExampleAddonV1
-materials
--<insert material files here>
-models
--<insert model files here>
It goes in the garrysmod/garrysmod directory, just drag the materials and models folder out of the 'ExampleAddonV1' folder, the materials and models folder will overwrite the already existing materials and models folder, but it won't delete anything, it'll only place the materials and models in the folders of the addon you're installing
If the folder structure is like this
ExampleAddonV2
-info.txt
-materials
--<insert material files here>
-models
--<insert model files here>
It goes in the garrysmod/garrysmod/addons directory, drag the 'ExampleAddonV2' folder into said directory
Not all addons with new models (like ragdolls, props, etc) will have spawn lists so after installing them and running Gmod, you'll need to open the spawn menu (Q is the default key) and click the [Browse] tab
