There are multiple executions in this game, some including guns which depending on if you have a gun from the category the execution is using will use that custom gun instead of a default one.

This execution uses the pistol category, so it should use only pistols for it, and I have a karambit, so it should technically use a default pistol, but this game broke it somehow, and now you can use the karambit and the gutter knife for pistol executions.