How about using blur?

ZStack {
    RoundedRectangle(cornerRadius: 5)
        .strokeBorder(Gradient(colors: [.blue, .red]), lineWidth: 2)
        .padding(20)
        .blur(radius: 4) // like shadow
    RoundedRectangle(cornerRadius: 5)
        .strokeBorder(Gradient(colors: [.blue, .red]), lineWidth: 2)
        .padding(20)
}