I realize there are people with studios where people may seek out a photographer to have their own G&N type photos done of themselves. It's very obvious how these people make money because the people are seeking the photographers.
However, more generally its paid models who are hired out to do G&N. There can be significant costs involved including hiring the model, hair and makeup, wardrobe and possible location and transporatation costs.
Do random people purchase these kind of photos? I can easily see how a single photo session can cost from $100 to thousands of dollars.