Here's how I got it to work for me today:
- Go to Wallet app
- Swipe up from the bottom to reveal "Edit Passes" and tap it
- Select "Scan Code"
- Focus on the QR code in the top left corner on the back of the gift card -- remove the silver peel off film first
- Wait for a notification to drop down from the top of the screen giving an option to open in Safari -- tap it
- Safari opens with an image of the gift card and shows the amount
- "Add" appears in the top right corner to add the card to Apple Wallet
I'll also mention that I tried this a bunch of times and I'd get the message to open Safari but it would just go to the Apple.com web site with no information about the gift card. Then, on a whim, I turned the phone off then back on and tried it and it worked. The world works in mysterious ways. . .
(iPhone 7 with the iOS 11.2 Public beta 2)