Hey there Teresita,
Welcome to planDisney and thanks so much for your question! It sounds like you great a great deal that'll help you have a fantastic visit to the Disneyland Resort.
Disney Gift Cards are so convenient since you can use them just about anywhere around the parks, and also a great way to stay on a budget during your visit.
Since gift cards are not automatically loaded to your Disney account, my guess is that you received your gift card information (a gift card number or barcode) from Costco - likely in your confirmation email. That is the first place I would check and if you're still having trouble finding it then you'll need to reach out to their customer service line to see how to access your gift card.
You're not able to save gift cards as a preferred payment method in the
Disneyland mobile app, so what I typically do is keep the gift card barcode handy to scan with my phone camera if I'm paying for something in the parks through the mobile app. Alternatively, if you're paying for something in person in the parks, you can also show the barcode to the Cast Member at the point of purchase as well.
Thanks again for your question, Teresita! I hope you have a fantastic visit to the Happiest Place on Earth.
Have a Magical Day,
Greg