I have an object, lets say it has a sine like shape which has a width of a few 10px, so that a gradiant color fill fits perfectly within the width. Now I want to fill the curved object along the centerline with such a gradiant color fill - is this possible with inkscape and if so, how?

You mean you want curved gradients?

http://wiki.inkscape.org/wiki/index.php ... _stroke.3F

I guess you want the last mentioned method, draw a curved path, duplicate it move it away and give it different stroke color (no fill) and use Effects > Generate from Path > Interpolate with the style option on and enough steps so you don't have gaps between the generated curves.