How to gift an app on the App Store for iOS?
Gift option is available under Share sheet for an app, in the App Store app in iOS. Also note that you cannot gift free items.

Instructions to gift an app, from the Apple Support document, Send App Store &amp; iTunes Gifts via email - From your iPhone, iPad, or iPod touch


  Gift an app or another item
  
  
  Open the App Store app, iTunes Store app, or Books app. If you're not signed in, sign in with your Apple ID.
  Find the item that you want to gift. (You can't gift a free item.)
  Find and tap the Gift button:
  
  
  In the App Store, tap the More button  in the upper-right corner of the app's product page, then tap Gift App .
  In the iTunes Store, tap the Share button , then tap Gift .
  In the Books app, tap Gift .
  
  




However, it appears that the gift option is currently not shown. I have observed this in various devices running iOS 12.3.1. It's likely due to an error or an implementation change from Apple's end.

To resolve this, your best bet is to get in touch with their support team.


Apple Support (@AppleSupport) | Twitter


You can also submit feedback to Apple:


Feedback Assistant