I have noticed the smell under these conditions"
1.) when I let them sit too long after killing but before eviscerating (need to be ready to gut soon after killing)= more bacteria buildup/decomp faster.
2.) or... when I have punctured intestines and not washed thoroughly.
In this case, you may even see a bit of blue coloring under the skin, and usually near the abdomen. This is the bacteria beginning to decompose the meat. If cooked (temps 150-degrees +), you will find the gaminess disappears/not there, and also all bacteria is killed.
A good comparison is the odor that you receive when opening a package of pork/picnic roast etc.. it just has a bit of sulfur odor to it.
Meat is chemically active (compounds), and this odor also will be more noticeable when you allow the bird to rest in the fridge in a sealed or covered container.
The only exception is that I keep these birds for myself and do not sell or give those away (bad idea to give someone a stinky bird).