Problem installing skins/addons

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since around the time i installed gfscape, i havent been able to succesfully install an addon(skins in this case). after downloading it shows as a .zip and all i can do is manually exract to its destined folder. after it downloads and i try to open it acts like a .vpk and start installing itself, but then says "'blahblah' is not a valid source engine add-on. Installation Failed"
im not the most pc savvy individual so i was just going to uninstall gfscape(i dont want to).

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any game. l4d2/cs:go had some skins that ive been trying to install. i know gfscape isnt needed, i just said that incase it was related. like i said the .vpk just show up as .zip and if i try to open them with the steam addon installer i get that error msg.

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anyone? ive tried uninstalling gfscape and i still have the same problem. in the downloads window(firefox) it shows as a .zip i remember before it showed the valve icon and showed as a .vpk. it still tries to install but always tells me its not a valid source engine addon