I can’t seem to install addons correctly into gmod 13 from garrysmod.org.
What do you mean? Are they not working at all or are there errors?
Nearly all addons from garrysmod.org are incompatible. You will have to update them.
But there’s more in garrysmod.org than the steam workshop.
The problem is that Lua changed and the files are no longer compatible with garrysmod13.