Hi, we're not able to replicate this error you're seeing. Try removing it from your subscriptions and then reinstalling it. If that didn't solve it, try checking the file integrity on Gmod by going to game properties from your steam page, then go to local files and hit the button "Verify integrity of game files".

Finally, if neither of these solutions didn't work, look back at your subscribed files, unsub from one at a time and check if that solved it. This is to make sure that none of your subscribed items are conflicting with this one. This might be a tedious task depending on how many items you already installed, which is why I offer this solution more as a last resort than anything else.

There's not much we can do from our end, sadly. This looks to be a client side thing. We did notice more people are having this problem, however it's not breaking on our end so we can't be sure what causes it.