Giving a custom style a colored gradient background

I'm not sure how exactly to integrate this with a stylesheet background, but here is a way to achieve gradients using Graphics:

Overlay@{
  Graphics[
   Raster@{List@@@Table[Blend["AvocadoColors",i],{i,0,1,.005}]},
   AspectRatio->1/2,
   ImageSize->Full],
  Graphics[
   Text@Style["Gradient!",FontSize->[email protected]],
   AspectRatio->1/2,
   ImageSize->Full]}

EDIT

I stumbled on a different way to do this that would be easier for simple layouts (tidied up with kguler's suggestion):

Graphics[{
  Polygon[
    {{0, 0}, {1, 0}, {1, 1}, {0, 1}}, 
    VertexColors -> {Yellow, Yellow, Green, Green}], 
  Text[
    Style["Gradient!", FontSize -> [email protected]], 
    Scaled[{.5, .5}]]}, ImageSize -> Full, AspectRatio -> 1/2]

In versions 11.1+, you can use LinearGradientImage and add the text as Epilog:

Show[LinearGradientImage["BlueGreenYellow", 6 {100, 50}], 
 Epilog -> {Text[Style["Gradient!", FontFamily -> "Old English Text MT",  
     FontSize -> [email protected]], Scaled[{.5, .5}]]}]

Replace "BlueGreenYellow" with {Black, Red, Orange, White} to get

Update: In versions 12.2.0+, gradient filling is available as a graphics directive:

Graphics[{LinearGradientFilling["BlueGreenYellow"], Rectangle[],
  Text[Style["Gradient!", FontSize -> [email protected]], Scaled[{.5, .5}]]}, 
 AspectRatio -> 1/2]