Something tells me that in the Dark Aether saga, the Warden likely is the owner of the Blundergat. Since its design was based off a blunderbuss and a Gatling gun, both weapons were around the American Civil War, and the Warden is a former Confederate soldier, so he would have experience with old-timey firearms. Especially Gatling guns, which were manufactured in 1862, a whole 3 years before the Warden was betrayed by Ashwood’s elites.
I believe he may have also used the Blundergat within the Dark Aether, when he was still a survivor before he took control of the dimension. Although why he would discard the gun is unknown, perhaps once he got his powers; The Warden felt that he didn’t need a gun anymore or that it ran out of ammo, but I’m convinced that he has to be the owner of the Blundergat.
But then again, I could be wrong and mistaken; this is only my guess and my theory of how the Blundergat fits into Paradox Junction.