shmerl: It's not so usable, since it doesn't even have GOG forum like a subproject. But if it would be there, then I could propose using it instead.
Making unofficial bug tracker will allow community making more categories. I'd make one just for the forum.
Yeah, that's a good point. It'd be nice if GOG expanded Mantis to allow more categories and more options such as the ability for reporters to reopen/close and make other status changes, and the ability for us to flag certain bugs/comments/file attachments as "private" if they contain sensitive personal information or security issues for example.
There are issues that I explicitly have avoided reporting in mantis due to lack of security context, not to mention the fact that they are running it over non-SSL and
https://letsencrypt.org/ has offered free SSL certs for 8+ months now so it's a no-brainer to just put everything on SSL these days. :)
It's also possible that making a public community ran bug tracker could influence them to either use it also, or more likely to add improvements to their own over time. Of course being unofficial too, they may not appreciate it or use it perhaps. If it's a community thing that the community finds useful though I don't see why not, but people should still use mantis and/or support for now if something is an issue they want GOG to explicitly be aware of too. :)