Don't worry! I will help! First after you download it, extract it to your desktop. Then open the folder. Now your going to want to go to your steam library and right click garry's mod. Then click properties. Then click the local files tab. Then click Browse local files. Go back to the folder and there should be 3 folders and a .txt file. drag the lua and particles straight into the current Garry's mod folder you have open. Then drag the last folder into the addon's folder. Then start your Garry's mod after all of that is done. You can delete the .txt file and extra folder after this. and it should work!

thanks man, all errors have been eliminated but htere are still a lot of models that are textureless (ex. the couches, some machinery, a few cranes, black mesa gluon cannon etc.), which bother me a little. Is there something I'm missing? I did exactly as you said, I put ATOM slim in the addon folder and "lua" and "particles" in the "garry's mod" folder, but this still happens.

No Because I have the same isssues. They might be textures from other packs that the developer did not include. Sorry I could not help you there.

it's fine dude

I belive you need Half life episode 2 textures for the rest. But I'm not fully sure.

I already have episode 2, still nothing

Then I don't know.

one solution that I found is installing black mesa models from the workshop

there are still some errors, like the door frames in the test chamber and that (♥♥♥♥♥♥♥) piece of machinery in the lab that I don't know where to find