308pirate is right on. The lead time isn't how long it takes to do the work, it is when your part comes up in line mostly. There are a couple things to keep in mind with lead times though.
One, it cuts both ways. For every person that would be ecstatic to get a call 2 months into a 6 month wait, there is another person that will not be happy. A lot of people budget so that they will have the money to pay for it at the end of the 6 months and if you call them at month 2 they won't have the money for another 4 months. But at the same time, anyone waiting with money in hand is going to be upset if they were told 6 months and they are on the 9th month of waiting. So any decent company is going to give you as accurate as possible lead time that they can, they are not going to say 2 months just to buffer it.
Two, I know for a fact that GAP tries very hard to hit the date quoted every time. If something gets held up and it is looking like it may be late then things shift, overtime hours are worked, and it is pushed to the front. This doesn't matter if you are a dealer ordering 20 guns or someone that sent a rifle in to get a muzzle brake put on; big or small order everything possible will be done to hit that date. So that means the people that ordered earlier and have a closer delivery date may push you back; but your gun may do the same to others. With lots of orders it has to be balanced out so that every single customer is getting a fair deal.
Now, as far as does it take two months, that will vary all the time based on where you are in line and what is in front, but GAP will not look at the date and say "we got two months, lets not bother with this one for another six weeks." Sometimes things are done earlier and if so then you will know either the same day or the next (if it gets finished late in the day) that it is done.