This is just an opinion by Bob Barr and he has no proof on this.
“This” being which point? Petraeus and gun control? There are plenty of other sources that say the same thing. Petraeus’ support of gun control and other liberal policies, like that of many flag officers that survived Obama’s purges, has been known for quite some time.
Here’s one you might consider more authoritative, but there are many, many more: http://www.wnd.com/2016/11/top-8-facts-you-dont-know-about-david-petraeus/
Besides, Petraeus is a lying, cheating scumbag and shouldn’t be anywhere near classified information ever again.