I am currently working with a builder who tells me that here in GA it is suggested that I place a humidifier on the first floor when having site finished hardwoods installed. That the humidity will help the floor from gapping in the winter when the gas heat is used. Is this the case or is it a case of the builder trying to get more **.

First off.
The flooring that is going to be layed down should be opened up and stacked to allow air to flow around it for a few days. A week is ideal.
This allows the moisture in the air to swell or shrink the flooring material to its natural state. The sub-floor and new flooing should be checked for moisture as well. 10% to 15% range should be about right. If one or the other is off that range do not install until they are close or match in moisture levels. Then the floor should be layed. Paying attention to the nails being set into the plywood and into the joists below for best fastening.
The floor will dry out some more as it ages and some gaps may develop But if properly installed these gaps should be far and few between and not an issue of appearance. If the floor is placed down with higher moisture levels as it drys it will shrink to much and gaps will develop.
He is sort of correct. Using a humidfier in the house can help prevent the floor from shrinking but if the house is new, There should be enough moisture in the home for a while before you would need to worry about this.