When I click on the "Publish" button in the "Publish" section, an error message appears with this text:

"Publishing item - opening connection to Steam Workshop.
Connection to Steam Workshop failed. This most likely means you need to login to Steam or you do not own the app or game to which this item belongs. "

I don't understand what the problem is, since I have Steam running and I bought the Garry's Mod ...
Can anyone help?