GOG switched to https across the site quite a while back, but there are still some remaining http turds remaining in their website code. If you visit the forums they contain a URL embedded in the page that links to the same page you're on using a fully pathed http link even when you're on https. This page itself contains:
<link rel="canonical" href="[url=http://www.gog.com/forum/general/the_what_did_just_break_thread_20"></head>]http://www.gog.com/forum/general/the_what_did_just_break_thread_20"></head>[/url];
We suggest you upgrade newer and better browser like:<a class="browser-link vdark_un" href="[url=http://google.com/chrome">Chrome</a>,<a]http://google.com/chrome">Chrome</a>,<a[/url] class="browser-link vdark_un" href="[url=http://www.mozilla.org/en-US/firefox/new/">Firefox</a>]http://www.mozilla.org/en-US/firefox/new/">Firefox</a>[/url];
(Not sure if the forum code will mangle any of that)
There are a variety of other URLs, offsite and on throughout the page that use http when https should be used. Both Google.com and mozilla.org for example have been https enabled for eons, so any URLs present should be https wherever possible whether on GOG or offsite where they officially support https.
On site, the best way to handle it is to not specify the URI, and write the URLs as:
//www.gog.com/whatever
Looking through the page I can see GOG is already using the non-specified URI syntax, however it isn't applied everywhere, leaving many hard coded http:// links to www.gog.com remaining. I discovered this because I use NoScript and everything works fine however I noticed in the "Recently Blocked" section of NoScript that "
http://www.gog.com" was blocked. Since the site is supposed to be entirely https now I wondered why GOG would be attempting to load Javascript over http on www.gog.com at all.
If they're using HSTS, these would be upgraded to https, however they're still site bugs that should be fixed, as should the offsite links that are http where https is supported for a long time as well.