FDA trials are only required to compete against a placebo. A drug is accepted if it is safe and more effective than a placebo in a double-blind clinical trial. That's it.
So they have no data to show with, unless they went above and beyond to get it. (Which should teach them a lesson about testing their drugs.)
You don't have to have head-to-head data to claim your drug is superior to existing therapies. Yes, head-to-head data is the best way to show it, but if you have two separate trials, each comparing to a placebo, you can draw conclusions about relative efficacy. It's not perfect, but in the case of Gilead, their drug blows away anything else currently on the market.
So they have no data to show with, unless they went above and beyond to get it. (Which should teach them a lesson about testing their drugs.)