Вот как это сделать в Swift 5:
extension NSImage {
convenience init?(gradientColors: [NSColor], imageSize: NSSize) {
guard let gradient = NSGradient(colors: gradientColors) else { return nil }
let rect = NSRect(origin: CGPoint.zero, size: imageSize)
self.init(size: rect.size)
let path = NSBezierPath(rect: rect)
self.lockFocus()
gradient.draw(in: path, angle: 0.0)
self.unlockFocus()
}
}