Hello, I have a variety of different Garters. If you feed him fish do so 2 to 3 times a week. Let him have his fill. You can also feed him earthworms or nightcrawlers (be careful of catching them if you have chemicals in your yard). "Redwigglers" are supposed to be bitter and somewhat toxic so most keepers recomend not useing them. Most Garters will also eat mice and yours may be big enough for pinkies. They may have to be scented at first however (just rub it on a goldfish). If you feed it pinkies once a week is fine. Garters also eat amphibians though this is usually impractical. Amphibians also carry parasites. Fish may also carry parasites but thawed frozen food should alleviate any worries. THe vitamin deficiancy (Thamine) caused by fish is generally associated with certain oily fishes, I have never heard of guppies or minnows implicated in this problem. Veggies are definatly not on the diet. I have heard of Garter snakes eating fish flavored cat food, but I have no experience with this nor do I have any knowlege of the nutritional issues involved. I hope this helps.
Good luck and enjoy your pet.