You can only get a gradient color if you:
- Use a gradient color on OSX Aqua, which has a more sophisticated coloring scheme that Tk plugs into partially, or
- Use a style that paints the background using a photo image that has a gradient in it, or
- Write a custom style engine that does everything itself.
The first is very platform specific (and I don't know the details), the second is taking you into the complex world of custom image styles, and the third involves deep voodoo that is definitely beyond what I know!