Это должно работать:
static let gradientStart = Color(red: 239.0 / 255, green: 120.0 / 255, blue: 221.0 / 255)
static let gradientEnd = Color(red: 239.0 / 255, green: 172.0 / 255, blue: 120.0 / 255)
var body: some View {
Rectangle()
.fill(LinearGradient(
gradient: .init(colors: [Self.gradientStart, Self.gradientEnd]),
startPoint: .init(x: 0.5, y: 0),
endPoint: .init(x: 0.5, y: 0.6)
))
.frame(width: 300, height: 200)
}