Используйте этот код в вашем методе UIView -drawRect:
size_t locationsCount = 2;
CGFloat locations[2] = {0.0f, 1.0f};
CGFloat colors[8] = {0.0f,0.0f,0.0f,0.0f,0.0f,0.0f,0.0f,0.75f};
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, colors, locations, locationsCount);
CGColorSpaceRelease(colorSpace);
CGPoint center = CGPointMake(self.bounds.size.width/2, self.bounds.size.height/2);
float radius = MIN(self.bounds.size.width , self.bounds.size.height) ;
CGContextDrawRadialGradient (context, gradient, center, 0, center, radius, kCGGradientDrawsAfterEndLocation);
CGGradientRelease(gradient);
это взято из здесь (строка 189)