Logically, that doesn't make any sense to me.

On December 31, 2007 – 2,293,157 prisoners were held in federal or state prisons or in local jails. (from http://ojp.usdoj.gov/bjs/prisons.htm, Bureau of justice statistics).

If half of those inmates are GT, that's 1,146,579.

In the general population, 3% are GT. There are 303,824,640 (according to https://www.cia.gov/library/publications/the-world-factbook/print/us.html) people in the US.
If 3% of the people in the country are gifted, there are 9,114,739 gifted people in the US.

That would mean that 13% (1M/9M) of all the gifted people in the US are in prison or jail. Hmmmmm. I'm having a hard time believing that.

If we just use prison numbers, I get 1,532,817 (same source) prisoners, 766,408 of whom are alegedly gifted. That's 8.5% of the gifted population in jail.