This too was bothering me, and I have found the solution navigate to \DMC Devi May Cry\DevilGame\Config then open this file in notepad DefaultInput.ini, next find this line: GUNSPECIAL= NTB_V_GAME2
I use a PS3 controller emulated as an xBox360 controller, for me I changed the binding to be the left shoulder evade button so my config reads: GUNSPECIAL= NTB_LEFT_SHOULDER
save the file after you have made the necessary changes to the file. Start up DmC and gun special should now be custom binded to whatever you set it as.