Hi Christy,
You can either add your Disney gift cards to your room account at check-in so that the funds are stored already on your account OR you can periodically stop by the Front Desk to pay off your expenses using the Disney gift cards OR you could wait until check-out to apply the gift cards. Lots of choices! If you use the last two choices, you'll have to put down a credit card to set up your account but you can pay it off with your Disney gift cards during or at the end of your trip.
We've used all three methods on past vacations to Walt Disney World. If you don't use your gift cards at check-in, be sure to keep them safely stored in your room safe. I like to review our expenditures every couple of days and pay off the balance periodically then there's no surprises at the end of our vacation and it keeps us more disciplined with our spending.
So, yes gift cards can added to your room account for purchases that you charge with your Key to the World card.
If you have funds left on the account from the Disney gift cards at the end of your stay, they will be reimbursed to you on a
Disney gift card. This differs from cash deposits in which the guest would have cash reimbursed to them.
If you need further clarification, just let me know!