iPad & iPhone
You can purchase a gift copy of the iPad or iPhone versions by following the instructions here: http://support.apple.com/en-jo/HT201783
Mac
The Mac App Store does not currently support gift purchases, but you can instead purchase an iTunes Gift Card that can be redeemed towards the purchase of the Mac version, as described here: https://www.apple.com/shop/buy-giftcard/giftcard
Windows
You can purchase a gift copy of the Windows version here: http://www.paprikaapp.com/buy/windows/?option=gift
Android
The Google Play Store does not currently support gift purchases, but you can instead purchase a Google Play Gift Card that can be redeemed towards the purchase of the Android version: https://play.google.com/intl/en-US_us/about/giftcards/