So, carbine lenght the gas past is at 7" of the barrel, the medium is 9".
But I’ve never seen any mention of the lenght of the gas port in a 20" barrel AR. I’ve always wondered about that.
Rifle gas system is about 12" from the barrel extension.
Did you mean the length of the gas tube or location of the FSB/ gas port? Off the top of my head I believe it is around 12".