I'm not sure what your DrawLayersToBitmap function does, but typically after making changes to the display characteristics you would call Invalidate to cause your window to redraw.
If you can post your GM_AreaStyle_t declaration we can check that as well. Osei's recommendation to check the sizes between C++ and C# is good to help verify your declaration is correct.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com