They used a combination.
@slattimer can probably answer this better.
The play that brought the spread to become the sexy offense was the ZONE read.
Now the spread system has evolved to use gap and zone schemes. The tough part is usually gap schemes are easier when you have a lead blocker (FB). So because you don't see much FBs on spread teams rosters you will see pullers and using TEs flexed to help lead. We saw Hickman do this quiet a bit last year. It was their short yardage package they went to. Then they ran that pop pass to him to keep teams honest that would have been an easy TD if Pieper doesn't get blown up. The play that everybody hated.
The thing I would be careful is labeling gap as smash mouth and not thinking zone isn't.
He played that hit the hole hard role at GA Tech then was in a 22 personnel gap scheme at Juco. He had to learn how to run in a spread zone scheme when he got here.
I really felt like I would have put Mills in the pistol more. I think that would have helped him out and kept some of the principals Frost likes.