The NU/OU game in 2001 is still tops in Gameday attendance, but it wasn't 20,000. See the tweet from Big 12 Conference.
The largest attendance for CGD is 22,144 at Kentucky last year. (This is for Basketball)
Around 20,000 were in attendance for Alabama vs Clemson for Football in 2008.
The listed # from Huskers was over 14,000 for 2007 (my bad). However, we don't hold the record anymore.