After you check out you will receive an email with a link to your gift card code. You can can forward the email wherever you want, or copy down the code directly.
You can enter a gift card code at checkout, in the same place you would add a discount code.
Unfortunately discount codes cannot be used on gift cards. They will apply to any other items in your cart, though.
Yep! You can use as many gift card codes as you need to when you check out.