A flutter widget that gives the given child a configurable gradient border (used and explained on https://www.flutterclutter.dev/flutter/tutorials/how-to-add-a-border-to-a-widget/2020/587/)

gradient_border_container.dart

import 'package:flutter/material.dart';

class GradientBorderContainer extends StatelessWidget {
  GradientBorderContainer({
    @required gradient,
    @required this.child,
    @required this.onPressed,
    this.strokeWidth = 4,
    this.borderRadius = 64,
    this.padding = 16,
    this.outlineWidth = 1,
    splashColor,
  }) :
  this.painter = _GradientPainter(
    gradient: gradient, strokeWidth: strokeWidth, borderRadius: borderRadius, outlineWidth: outlineWidth
  ),
  this.splashColor = splashColor ?? gradient.colors.first;

  final _GradientPainter painter;
  final Widget child;
  final VoidCallback onPressed;
  final double outlineWidth;
  final double strokeWidth;
  final double borderRadius;
  final double padding;
  final Color splashColor;

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: painter,
        child: InkWell(
          highlightColor: Colors.transparent,
          splashColor: splashColor,
          borderRadius: BorderRadius.circular(borderRadius),
          onTap: onPressed,
          child: Container(
            padding: EdgeInsets.all(padding + strokeWidth + outlineWidth),
            child: IntrinsicWidth(
                child: child
            )
          ),
        ),
    );
  }
}

class _GradientPainter extends CustomPainter {
  _GradientPainter({this.gradient, this.strokeWidth, this.borderRadius, this.outlineWidth});

  final Gradient gradient;
  final double strokeWidth;
  final double borderRadius;
  final double outlineWidth;
  final Paint paintObject = Paint();

  @override
  void paint(Canvas canvas, Size size) {
    if (outlineWidth > 0) {
      _paintOutline(outlineWidth, size, canvas);
    }

    Rect innerRect = Rect.fromLTRB(
        strokeWidth, strokeWidth, size.width - strokeWidth, size.height - strokeWidth
    );
    RRect innerRoundedRect = RRect.fromRectAndRadius(innerRect, Radius.circular(borderRadius));

    Rect outerRect = Offset.zero & size;
    RRect outerRoundedRect = RRect.fromRectAndRadius(outerRect, Radius.circular(borderRadius));

    paintObject.shader = gradient.createShader(outerRect);
    Path borderPath = _calculateBorderPath(outerRoundedRect, innerRoundedRect);
    canvas.drawPath(borderPath, paintObject);
  }

  void _paintOutline(double outlineWidth, Size size, Canvas canvas) {
    Paint paint = Paint();
    Rect innerRectB = Rect.fromLTRB(
        strokeWidth + outlineWidth,
        strokeWidth + outlineWidth,
        size.width - strokeWidth - outlineWidth,
        size.height - strokeWidth - outlineWidth
    );
    RRect innerRRectB = RRect.fromRectAndRadius(innerRectB, Radius.circular(borderRadius - outlineWidth));

    Rect outerRectB = Rect.fromLTRB(-outlineWidth, -outlineWidth, size.width + outlineWidth, size.height + outlineWidth);
    RRect outerRRectB = RRect.fromRectAndRadius(outerRectB, Radius.circular(borderRadius + outlineWidth));

    Path borderBorderPath = _calculateBorderPath(outerRRectB, innerRRectB);
    paint.color = Colors.black;
    canvas.drawPath(borderBorderPath, paint);
  }

  Path _calculateBorderPath(RRect outerRRect, RRect innerRRect) {
    Path outerRectPath = Path()..addRRect(outerRRect);
    Path innerRectPath = Path()..addRRect(innerRRect);
    return Path.combine(PathOperation.difference, outerRectPath, innerRectPath);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

partial_border_container.dart

class PartialBorderContainer extends StatelessWidget {
  PartialBorderContainer({
    @required gradient,
    @required this.child,
    @required this.onPressed,
    this.strokeWidth = 4,
    this.padding = 16,
    splashColor,
  }) :
  this.painter = PartialPainter(
      strokeWidth: strokeWidth, gradient: gradient
  ),
  this.splashColor = splashColor ?? gradient.colors.first;
  final PartialPainter painter;
  final Widget child;
  final VoidCallback onPressed;
  final double strokeWidth;
  final double padding;
  final Color splashColor;

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: painter,
      child: InkWell(
        highlightColor: Colors.transparent,
        splashColor: splashColor,
        onTap: onPressed,
        child: Container(
            padding: EdgeInsets.all(padding + strokeWidth),
            child: IntrinsicWidth(
                child: child
            )
        ),
      ),
    );
  }
}

class PartialPainter extends CustomPainter {
  PartialPainter({this.strokeWidth, this.gradient});

  final Paint paintObject = Paint();
  final double strokeWidth;
  final Gradient gradient;

  @override
  void paint(Canvas canvas, Size size) {
    Rect topLeftTop = Rect.fromLTRB(0, 0, size.height / 4, strokeWidth);
    Rect topLeftLeft = Rect.fromLTRB(0, 0, strokeWidth, size.height / 4);

    Rect bottomRightBottom = Rect.fromLTRB(size.width - size.height / 4, size.height - strokeWidth, size.width, size.height);
    Rect bottomRightRight = Rect.fromLTRB(size.width - strokeWidth, size.height * 3 / 4, size.width, size.height);

    paintObject.shader = gradient.createShader(Offset.zero & size);

    Path topLeftPath = Path()
      ..addRect(topLeftTop)
      ..addRect(topLeftLeft);

    Path bottomRightPath = Path()
      ..addRect(bottomRightBottom)
      ..addRect(bottomRightRight);

    Path finalPath = Path.combine(PathOperation.union, topLeftPath, bottomRightPath);

    canvas.drawPath(finalPath, paintObject);
  }
  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

show_borders.dart

import 'gradient_border_container.dart';
import 'partial_border_container.dart';

class Borders extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
          child: Center(
            child: Column(
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                SizedBox(width: 0, height: 16),
                _getButtonWithGradientRect(),
                SizedBox(width: 0, height: 16),
                _getButtonWithGradientRRect(),
                SizedBox(width: 0, height: 16),
                _getDangerButton(),
                SizedBox(width: 0, height: 16),
                _getDangerButton2(),
                SizedBox(width: 0, height: 16),
                _getButtonWithPartial()
              ],
            ),
          )
      ),
    );
  }

  GradientBorderButtonFinal _getButtonWithGradientRect() {
    return GradientBorderButtonFinal(
      strokeWidth: 4,
      radius: 0,
      padding: 20,
      gradient:
      LinearGradient(colors: [Colors.redAccent, Colors.orangeAccent]),
      child: Text('GRADIENT', style: TextStyle(fontSize: 32)),
      onPressed: () {},
    );
  }

  GradientBorderButtonFinal _getButtonWithGradientRRect() {
    return GradientBorderButtonFinal(
      strokeWidth: 4,
      radius: 54,
      padding: 20,
      gradient:
      LinearGradient(colors: [Colors.green, Colors.blue]),
      child: Text('ROUNDED', style: TextStyle(fontSize: 32)),
      onPressed: () {},
    );
  }

  GradientBorderButtonFinal _getDangerButton() {
    return GradientBorderButtonFinal(
      strokeWidth: 16,
      radius: 16,
      gradient: LinearGradient(
        begin: Alignment.topLeft,
        end: Alignment(-0.2, -0.4),
        stops: [0.0, 0.25, 0.25, 0.5, 0.5, 0.75, 0.75, 1],
        colors: [
          Colors.yellow,
          Colors.yellow,
          Colors.black,
          Colors.black,
          Colors.yellow,
          Colors.yellow,
          Colors.black,
          Colors.black,
        ],
        tileMode: TileMode.repeated,
      ),
      child: Text('DANGER!',
          style: TextStyle(fontSize: 32, fontWeight: FontWeight.bold)),
      onPressed: () {},
    );
  }

  GradientBorderButtonFinal _getDangerButton2() {
    return GradientBorderButtonFinal(
      strokeWidth: 16,
      radius: 16,
      outlineWidth: 1,
      gradient: LinearGradient(
        begin: Alignment.topLeft,
        end: Alignment(-0.2, -0.4),
        stops: [0.0, 0.25, 0.25, 0.5, 0.5, 0.75, 0.75, 1],
        colors: [
          Colors.pinkAccent,
          Colors.pinkAccent,
          Colors.white,
          Colors.white,
          Colors.pinkAccent,
          Colors.pinkAccent,
          Colors.white,
          Colors.white,
        ],
        tileMode: TileMode.repeated,
      ),
      child: Text('TRAIN CROSSING!',
          style: TextStyle(fontSize: 32, fontWeight: FontWeight.bold)),
      onPressed: () {},
    );
  }

  Widget _getButtonWithPartial() {
    return PartialBorderContainer(
      strokeWidth: 4,
      padding: 20,
      gradient:
      LinearGradient(colors: [Colors.green, Colors.blue]),
      child: Text('PARTIAL BORDER', style: TextStyle(fontSize: 32)),
      onPressed: () {},
    );
  }
}