You can use a CAGradientLayer to get the gradient effect, and use the CAShapeLayer as a mask.
e.g.:
- (void)viewDidLoad
{
[super viewDidLoad];
int radius = 100;
CAShapeLayer *arc = [CAShapeLayer layer];
arc.path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(100, 50) radius:radius startAngle:60.0 endAngle:0.0 clockwise:YES].CGPath;
arc.position = CGPointMake(CGRectGetMidX(self.view.frame)-radius,
CGRectGetMidY(self.view.frame)-radius);
arc.fillColor = [UIColor clearColor].CGColor;
arc.strokeColor = [UIColor purpleColor].CGColor;
arc.lineWidth = 15;
CABasicAnimation *drawAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
drawAnimation.duration = 5.0; // "animate over 10 seconds or so.."
drawAnimation.repeatCount = 1.0; // Animate only once..
drawAnimation.removedOnCompletion = NO; // Remain stroked after the animation..
drawAnimation.fromValue = [NSNumber numberWithFloat:0.0f];
drawAnimation.toValue = [NSNumber numberWithFloat:10.0f];
drawAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
[arc addAnimation:drawAnimation forKey:@"drawCircleAnimation"];
CAGradientLayer *gradientLayer = [CAGradientLayer layer];
gradientLayer.frame = self.view.frame;
gradientLayer.colors = @[(__bridge id)[UIColor redColor].CGColor,(__bridge id)[UIColor blueColor].CGColor ];
gradientLayer.startPoint = CGPointMake(0,0.5);
gradientLayer.endPoint = CGPointMake(1,0.5);
[self.view.layer addSublayer:gradientLayer];
//Using arc as a mask instead of adding it as a sublayer.
//[self.view.layer addSublayer:arc];
gradientLayer.mask = arc;
}