If you buy a gift card directly from
Shop Disney, there will be a button link in the email to add it to Apple Wallet. There doesn’t seem to be a way to do it with physical gift cards or e-gift cards purchased from third party web sites like Target.
Since I had a lot of discounted gift cards I bought from BJs, I bought one $25 gift card from Shop Disney that I added to Apple Wallet. Then I transferred the balance from my other gift cards using the Disney Gift Card web site.
It was so convenient to use in the parks!