This Script convert Sketch gradient to CALayer Swift Code. Open Sketch, select one layer, press control + shift + K paste code to run.

sketch_gradient_to_ios_swift_code.js

console.log('Select Layer Then Run Script.');

var sketch = require('sketch');
var UI = require('sketch/ui');

var document = sketch.getSelectedDocument();

var selectedLayers = document.selectedLayers;
var selectedCount = selectedLayers.length;

function hexToRGBA(hex) {
  var result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
  return result ? {
    r: parseInt(result[1], 16),
    g: parseInt(result[2], 16),
    b: parseInt(result[3], 16),
    a: parseInt(result[4], 16)
  } : null;
}

let precesion = 10000

function parseGradient(layerGradient) {

  let colorItems = ""
  let locationArray = ""

  layerGradient.stops.forEach(elm => {
    let rgbaColor = hexToRGBA(elm.color)
    colorItems += `UIColor(red: ${rgbaColor.r}/255.0, green: ${rgbaColor.g}/255.0, blue: ${rgbaColor.b}/255.0, alpha: ${rgbaColor.a}/255.0), `
    locationArray += `${Math.round(elm.position*precesion)/precesion}, `
  });

  var codeTemplate = `
import Foundation
import UIKit

class LinearGradientLayer: CALayer {
    
  required override init() {
      super.init()
      needsDisplayOnBoundsChange = true
  }
  
  required init?(coder aDecoder: NSCoder) {
      super.init(coder: aDecoder)
  }
  
  required override init(layer: Any) {
      super.init(layer: layer)
  }
  
  let colors = [${colorItems}].map(\{ \$0.cgColor \})
  
  override func draw(in ctx: CGContext) {
      ctx.saveGState()
      let colorSpace = CGColorSpaceCreateDeviceRGB()
      let locations: [CGFloat] = [${locationArray}]
      let gradient = CGGradient(colorsSpace: colorSpace, colors: colors as CFArray, locations: locations)!
      
      ctx.drawLinearGradient(gradient, start: CGPoint(x: ${Math.round(layerGradient.from.x*precesion)/precesion}*bounds.width, y: ${Math.round(layerGradient.from.y*precesion)/precesion}*bounds.height), end: CGPoint(x: ${Math.round(layerGradient.to.x*precesion)/precesion}*bounds.width, y: ${Math.round(layerGradient.to.y*precesion)/precesion}*bounds.height), options: [.drawsBeforeStartLocation, .drawsAfterEndLocation])
  }
}
`;

  return codeTemplate
}

if (selectedCount === 0) {
  UI.alert('No layers are selected', 'Select one gradient filled layer and rerun.')
} else {
  selectedLayers.forEach(function (layer, i) {
    console.log((i + 1) + '. ' + layer.name);
    let layerGradient = layer.style.fills[0].gradient;

    console.log(layerGradient)

    UI.getInputFromUser(
      "Convert Result",
      {
        initialValue: parseGradient(layerGradient),
        numberOfLines: 12
      },
      (err, value) => {
        if (err) {
          // most likely the user canceled the input
          return;
        }
      }
    );

  });
}

This script convert Sketch gradient layer to CALayer. Insert this layer by view.layer.insertSublayer(gradientLayer, at: 0).

Select a layer which contains one gradient fill, then press control + shift + K, paste code below and run. you will see:

And on iPhone it looks like: