Simple.
Create a VERSION constant in one of your scripts (or probably a new one), and assign it a value.
Host a version file on the location of your choice on the public Internet. Gist or Dropbox is great for that. In the file, write nothing more than the value that you assigned VERSION.
Then, run an http.Get() on your URL in Garry's Mod, probably when a GUI is created or on a hook of some type.
In the success callback, compare the response of http.Fetch() to the VERSION constant's value. If the same, up to date. If different, outdated. Handle as you wish.