Your first example domain is actually a URL. The
https part is the scheme used to connect to the following domain, Pi-hole will never see this part.
You also don't need to match the entire domain. Just matching a substring will work.
Thirdly, to match a literal dot (
.) you need to escape it (
\.), since the dot itself is used as a wildcard that matches any character.
You can simplify your regex to
$ match the start and end if a line. That ensures you do not match nefarious domains like xxxgameserver.comyyy, but do match subdomains like zzz.gameserver.com (notice the dot after the subdomain).