If you look at the wiki for Gmod gui.Openurl as linked in a previous post, a year ago, someone posted that https isn't working.
I've no idea if that's been fixed, as I don't track Gmod update change logs.
Additionally, Source HTML interpreter (which, Gmod uses) is extremely picky about the html and urls it will view.
That being said, I just tried changing my Steam client to BIG PICTURE mode, went to the "web", "new tab", pasted in the URL you provided, and got an error 105 - can't connect to server.
Might be something wrong with your URL, or, if it works in a normal browser (I didn't test), then its something with Source HTML/browser.