To answer about DVC discounts, I would encourage you to download the Mouseperks app or pin the website to your phone. It will show you which discount is accepted where, the %, & any restrictions such as time of day.
To answer about GC, I use them all the time, you can pay for meal in full including the tip to a GC or you can for partial payments with multiple forms of payment.
I use my Disney Visa to buy my GC so I then have my DV $$$ to spend by the time we arrive.