The following stacked arrangement produces an effect that is close enough to what I wanted.

Stack(
      children: [
        Container(
          width: double.infinity,
          height: double.infinity,
          decoration: const BoxDecoration(
            gradient: LinearGradient(
              begin: Alignment.topCenter,
              end: Alignment.bottomCenter,
              colors: [
                Color.fromARGB(255, 4, 75, 211),
                Color.fromARGB(255, 2, 39, 109),
              ],
            ),
          ),
        ),
        Container(
          decoration: BoxDecoration(
            gradient: RadialGradient(
              center: Alignment.center,
              radius: 1,
              colors: [
                Colors.white.withValues(alpha: 0.6),
                Colors.transparent,
              ],
            ),
          ),
        ),
      ],
    );