He normally eats medium to large fuzzies.
When cooking, the heat changes the protein and carbohydrate structure of the food, allowing for easier digestion. The thing is, people have theoretically evolved to eat cooked food, and I don't know if the garter's digestive system would be able to deal with the changes.
You also might be interested to know that a while back, when I was a bit short on F/T and had to feed my larger snakes fresh killed, I was taking the last adult mouse out of the tank, and putting the same garter (already fed) into it so he wouldn't get nippy with his tank mates, and he leaned over and bit the adult mouse right on its rear. Again, had to substitute my finger for the mouse to the snake, while dealing with a rather angry mouse.