How to recover the account after switching from Android to iOS

Backflip has instructions for this in their Android FAQ.

DragonVale park progress is stored on our servers for Android devices when the park is associated with a Facebook account. If you would like to associate your Local park with your Facebook account, please follow these steps (please note that this is a permanent choice and once you associate your Local park to your Facebook account, this cannot be undone):

1. Launch DragonVale
2. After DragonVale has loaded, tap on Options at the bottom of the screen
3. In the Options menu, tap on Login with the Facebook icon at the bottom. You will be directed to log into Facebook
4. You will receive a prompt asking if you want to continue playing your Local park or Facebook, select Facebook. Your Local park should now be associated with your Facebook account
5. If you receive an additional prompt after selecting Facebook, you likely will want to pick the higher level park (this will only happen if you have played DragonVale with your Facebook account in the past)

Please note that you must log into Facebook to access your saved park if for some reason you do not remain logged into Facebook in DragonVale.

Yes, gems can be gifted cross platform. However, because the Android version is not connected to iOS's GameCenter at all, your friend on Android must be friends with you on Facebook in order to be able to gift you gems and vice versa. Facebook is the only bridge between the two platforms. I use the Android version and I have 2 friends on iOS and one friend on Android and I have no issues gifting to them.

As a side note, the Android friending is a little buggy still. It took some time before my friends showed up after adding them through Facebook, and Dragonvale will still, on occasion, say that my Friends list is completely empty and/or have specific friends not show up on it randomly. Restarting the app will sometimes fix it, but it occasionally takes more time to fix itself. If this happens, don't worry about messing with Facebook or anything, your friends have not been lost, it's just the app being buggy.