The Legendary Showdown: Machines & Magic game and some stretch goal rewards.
Legendary Showdown: Machines and Magic is a real-world card game by Killer Robot Games that combines the worlds of Girl Genius & Gunnerkrigg Court “to create comically epic battles!” The Kickstarter compaign to fund the creation of this game ran from February 18 to March 17, 2015. The campaign was quite successful, raising $194,999 USD (nearly four times the initial goal) with 2,872 backers contributing.
The campaign was mentioned at the bottom of the page for February 18, 2015, ✣ but this reference is no longer present on the comic website; it was removed when the campaign ended.
Professora Foglio announced receiving a copy of the game on the now defunct Girl Genius Adventures blog in an entry dated January 31, 2016. (This blog is no longer reachable, so a link to an archived copy of the entry is provided.)
The Kickstarter campaign was also announced on Facebook.
Alas, the Killer Robots Games website is also no longer reachable and this game and other games and merchandise produced by this company no longer seem to be available.