Man, watching this play again, Gay MADE that stuff.
If the G manages that reach block and brings Gay inside, the FB gets through clean w/ a dead line at Sorensen. The HB follows him and if the FB seals off Sorensen, that play may have broken for 6 because the slot receiver had gotten inside position on Mathieu.
Just a really really good play by Gay to completely blow that play up. Not only did he keep the G from creating a lane at all, he even occupied the FB in the process. He forced that run back to the A gap where Hitchens (and I think Wilson) were there to snuff it out.
That's really good run D in a lot of ways but if Gay doesn't do his job really well there, it's a potential big play. Also a really nice call dialed up from Spags to put Gay in that run blitz and completely destroy the G's ability to get the reach made. If that's not a run blitz there, the G may have gotten position on him and been able to ride him inside.