Allow me to concede your point while explaining why I have the perception that I have about Tech's difficulty with defense.
First, scoring statistics during the Gaily years seem to point to a robust defense. Average number of points given up by Tech during his tenure are as follows:
2002: 20.5 per game
2003: 20.4 per game
2004: 17.4 per game
2005: 18.5 per game
2006: 19.2 per game
2007: 20.8 per game
But here is the fly in the ointment for me. It just so happens that 2006-2008 marked a sustained down turn in overall college offensive performance due to certain rule changes and other factors. A brief description of this can be found in
https://infogr.am/The-Offensive-Evolution-of-College-Football
So it appears that Tech had its best defensive performance commensurate with a time of relatively poor offensive performance by opposing teams.
Whether or not that is a valid argument, my
perception of Tech having perennial defensive problems is in part due to the fact that Gailey could never beat Georgia and this almost always had to do with inability to get key stops on defense. Likewise, each of Gailey's years his teams seemed to give up an avalanche of points to random teams at inopportune moments. A few examples would be .....
2002 -51 points to Georgia, 2003 -39 to Clemson and 41 to Duke, 2004 -30 to Virginia, 34 to UNC, 34 to Va Tech, 2005 -51 to Va Tech, 38 to Utah, 2006 -31 to Clemson, 38 to West Virginia, 2007 - 28 to Maryland, 31 to UGA, 40 to Fresno State.
Of course this is cherry picking key games but if you are a fan living and dying with the team these are the kinds of scores that cause you to curse the defense. Especially since these scores all happened in years when teams were not scoring as much as they do today.
For what is is worth, after 2008, overall college offensive production began to accelerate again with 2012 being the first year that the average Divion 1A football team gained more than 400 yards of offense per game.