Gradient Fill using Graphics in Mathematica

Question

How could I create the following using Rectangle[] in Graphics[] ?

Answer 1

Using Polygon , you can

Graphics[{EdgeForm[Black], 
  Polygon[{{0, 0}, {3, 0}, {3, 1}, {0, 1}}, 
   VertexColors -> {White, Red, Red, White}]}]

Answer 2

Also:

Graphics[Raster[{Range[100]/100}, ColorFunction -> (Blend[{White, Red}, #] &)], 
 AspectRatio -> .3, 
 Frame       -> True, 
 FrameTicks  -> None]