wrapping Scaffold with Container for gradient background, How to set gradient to container background in flutter?

You can also add a gradient to the AppBar like this,

new Scaffold(
      appBar: AppBar(
        title: Center(child: Text('Awesome AppBar')),
        flexibleSpace: Container(
          decoration: new BoxDecoration(
            gradient: new LinearGradient(
                colors: [
                  const Color(0xFF3366FF),
                  const Color(0xFF00CCFF),
                ],
                begin: const FractionalOffset(0.0, 0.0),
                end: const FractionalOffset(1.0, 0.0),
                stops: [0.0, 1.0],
                tileMode: TileMode.clamp),
          ),
        ),
      ),
      body: ...,
    );

LinearGradient parameters:

• colors: an array of the colors to be used in the gradient, in this case, two shades of blue.
• begin, end: position of the first color and the last color, in this case,
• FractionalOffset allows us to treat the coordinates as a range from 0 to 1 both for x and y. As we want an horizontal gradient, we use same Y for both measures, and the x changes from 0.0 (left) to 1.0 (right).
• stops: this array should have the same size than colors. It defines how the colors will distribute. [0.0, 1.0] will fill it from left to right. [0.0, 0.5] will fill the colors from left to half bar, etc.
• tileMode: what to do if the stops do not fill the whole area. In this case, we added clamp (it will reuse the last color used), but as our gradient goes from 0.0 to 1.0, it’s not really necessary.

Hope this helps.

In Your Code - Under Scaffold add - backgroundColor: Colors.transparent,

child: Scaffold(
        backgroundColor: Colors.transparent,
        appBar: AppBar(
        ..

Simply add FlexibleSpace in AppBar and Decorate the container. Then the app bar is a gradient in the background.

appBar: AppBar(

      title: Text('Flutter Demo'),
      flexibleSpace: Container(
        decoration: BoxDecoration(
          gradient: LinearGradient(
            begin: Alignment.centerLeft,
            end: Alignment.centerRight,
            colors: <Color>[
              Colors.red,
              Colors.blue
            ],
          ),
        ),
      ),
    ),