It looks like the error is coming from your normals map. The stepped gradient is that purple area light purple area on the door. Also, don't use 32-bit TGA for your normals map. You only use 32-bit when you have an alpha mask. Normals don't need an alpha channel. Go with 24-bit and don't compress anything.
Came across a problem. I have a stepped gradient look in marmoset, from my normal map. I was wondering if this problem is fixable? I thought this would be a problem if it was a compressed JPG or something, but its a 32 bit TGA...