It is not possible to apply a gradient fill to some Models even though it is possible to choose a color fill.
Models which cannot have a gradient fill are:
Timeline
Table
ClarisImpact will beep at you if you attempt to fill one of these Models with a gradient. All other Models will allow gradient fills.
Workaround:
Change your Timeline to a transparent fill (first choice on the pattern palette) and then draw a rectangle that matches the dimensions of the Timeline. Fill the rectangle with a gradient and send it behind the Timeline.