Hi there, MaryAnn!
You most certainly can use your
Disney Gift Cards as currency for tips. The cards are like cash, and you'll just need to make sure to have it with you at all times, as it cannot be attached to your Key to the World.
After a meal, you'll receive a receipt with your remaining entitlements on it as well as a line for gratuity. At this point, simply write in the amount of gratuity you'd like to offer, and submit your Gift Car as you would any credit or debit card following a meal. The amount will be debited from your Gift Card balance, and you will be able to see your remaining balance on your final receipt from the restaurant.
The same idea applies at the Bibbidi Bobbidi Boutique.
If you'd like to leave something for Mousekeeping, you will need to make arrangements with the Cast Members at the front desk. It can be done, but you will need to specify an amount and the Cast member will be able to debit that amount and apply it to Mousekeeping accordingly.
Have a wonderful trip!
See you later, celebrator,
-Deborah B.