It will look glossy as seen in that image on iOS 6
and lower OSs. iOS 7
is now flat.
Now if you wish to create your own UIAlertView
by subclassing it. Theres a few tutorials that will teach you how to. but you'll need to create your own graphics and will cause a extra programming.
All your custom drawing will have to be inside the drawRect
or overriding the various elements in the UIAlertView
.
I would look at the way this github project does it's own customization to the UIAlertView
https://github.com/Sumi-Interactive/SIAlertView
For example: