Gravy powder sounds interesting; you might check the ingredient list to make sure there isn't anything that isn't good for making soap. I do think you might have an issue with the gummy starch creating gel blobs in the soap. You can read some threads on here where people had blob issues with rice flour, for instance - another gummy substance that can be used to thicken gravies. Maybe give it a try in a small batch?
I have a bunch of carob powder, purchased before I learned that I don't like carob. 🤮 It is very similar to using cocoa powder - makes quite a nice brown, and I believe the starch in it contributes to enhanced bubbles. It does require lots of mixing to make sure there are no spots.