To add gift cards from other stores, just find their gift card page and add it the same way you add an item. Here's an example.
Leave the quantity blank (remove the 1) and add a zero to the price box. That makes it so there’s no limit to how many gift cards can be reserved and they can choose the amount they want to give.
If you’re adding an e-gift card that has to be emailed to you, make sure you include your email address in the note box so gift-givers know where to send it.