Things have changed and be simpler now for apps using the new shopify functions payment customizations api.
Search "payment" that customize checkout
https://apps.shopify.com/search?q=payment&features%5B%5D=checkout
if you can find a payments restriction app in the app store that works at the payment+product level;
Bascially it hides available payment methods in the payment step , preferably find one also able to surface some sort of message informing the user of the state of their cart and why it's valid/invalid.
It's still a niche need, and may need to be custom built if there are other exact requirements.
Otherwise this is the type of thing that generally required custom development either directly on the api(shopify Plus stores only) , or through a service like rise.ai , or possibly using workarounds using draft orders.
Alternative create second redemption store
It can also be saner to have a second store if no app/service meets the need: Because of the restrictions in modifying the checkout and the difficulties in enforcing this with only theme customizations along with adjusting the messaging on all channels, things like discounts, updating all your notifications etc.
From the second store either embed the gift-card buy-buttons on to main site,
Then redirect the first sites eligible carts of digital-goods to the gift-card redemption stores checkout using permalinks or draft orders.
Or, if the store is on shopify Plus, use automations to create the gift card on the redemption store now that gift card api is generally available.
If you need something like a shopify functions based app built, or a redemption store explored then contact me by mail for services.
ALWAYS please provide context, examples: store url, theme name, post url(s) , or any further detail in ALL correspondence.
Contact info in signature.
Good Hunting.
