How do I gift a Pro subscription on iOS?

It's currently not possible to gift a subscription with Apple, but you can give an App Store Gift Card.

When you send an Apple Gift Card, the recipient can use it to pay for items from the App Store. 

Follow these steps on your iPhone to send :

1. Open the App Store app.
2. Tap your photo or the sign-in button at the top of the screen. If you’re not signed in, sign in with your Apple ID.
3. Tap “Send Gift Card by Email.”
4. Enter your recipient’s email address. If you want, add a message.
5. Tap the amount that you want to gift, or tap Other to enter a different amount.
6. By default, the gift is sent immediately after you buy it. If you want to schedule the gift to send later, tap Today, then set the date that you want the gift to send.
7. Tap Next.
8. Select a theme for your gift, then tap Next.
9. Tap Buy, then tap Buy Now to confirm.

More information from Apple.

Once the receiver has gotten your gift, all they need to do is use it to subscribe to Pro.