This would mean I'm the only person allowed to sell a work based on my own GPL code on Steam. And also that if I merge any pull-request from someone else I'd loose that right, unless I get permission from each contributor.
But they also say "The author [can] decide that what the Steamworks SDK does is just a communication with a service that does not invoke the copyleft requirement of the GPL".
And then I'm wondering if this really only applies to "the author" or to anyone?