Photoshop style gradients in Core Graphics via use of CGShader (with thanks to @jernejstrasner, see his explanation of why your gradients might not look how your designers intended http://jernejstrasner.com/2014/01/09/smooth-gradients-ios.html) ⚠️ see comments for notes

gistfile1.swift

func SineEaseInOutLinearAverage(x: Double) -> Double {
    var easeInOutSine = ((cos(M_PI * x) - 1) / -2)
    return (easeInOutSine + x) / 2
}

func ShadingFunctionCreate(startColor: NSColor, _ endColor: NSColor, _ slopeFunction: (Double) -> Double) -> (UnsafePointer<CGFloat>, UnsafeMutablePointer<CGFloat>) -> Void {
    return { inData, outData in
        let q = CGFloat(slopeFunction(Double(inData[0])))
        outData[0] = startColor.redComponent + (endColor.redComponent - startColor.redComponent) * q
        outData[1] = startColor.greenComponent + (endColor.greenComponent - startColor.greenComponent) * q
        outData[2] = startColor.blueComponent + (endColor.blueComponent - startColor.blueComponent) * q
        outData[3] = startColor.alphaComponent + (endColor.alphaComponent - startColor.alphaComponent) * q
    }
}

func ShadingCallback(infoPointer: UnsafeMutablePointer<Void>, inData: UnsafePointer<CGFloat>, outData: UnsafeMutablePointer<CGFloat>) -> Void {
    var info = UnsafeMutablePointer<Gradient>(infoPointer).memory
    info.shadingFunction(inData, outData)
}

struct Gradient {
    let startColor : NSColor
    let endColor : NSColor
    let from : CGPoint
    let to : CGPoint
    let colorSpace = NSColorSpace.genericRGBColorSpace().CGColorSpace!

    init(startColor: NSColor, endColor: NSColor, from: CGPoint, to: CGPoint) {
        self.startColor = startColor
        self.endColor = endColor
        self.from = from
        self.to = to
    }
    
    lazy var shadingFunction : (UnsafePointer<CGFloat>, UnsafeMutablePointer<CGFloat>) -> Void = {
        return ShadingFunctionCreate(self.startColor, self.endColor, SineEaseInOutLinearAverage)
    }()
    
    lazy var CGFunction : CGFunctionRef? = {
        var callbacks = CGFunctionCallbacks(version: 0, evaluate: ShadingCallback, releaseInfo: nil)
        return CGFunctionCreate(&self, 1, [0, 1], 4, [0, 1, 0, 1, 0, 1, 0, 1], &callbacks)
    }()
    
    lazy var CGShading : CGShadingRef! = CGShadingCreateAxial(self.colorSpace, self.from, self.to, self.CGFunction, false, false)
}

Hi, I have an update for Swift 5 on iOS.
https://gist.github.com/LiLejia/3b67b5d3e0a83eb3ef251533e52a0889