That's not long for a new garter to go without eating...especially THIS time of year. Try offering small bait store minnows or rosies from a petstore (feeders). Offer them still wiggling and hold them carefully right in front of the snake's face (take it slow...don't jam it right in there).
Is this a wild caught or a captive born? Either way, it could be slowed down for the season. One of my garters went 2 months without eating this winter. So I put him it the basement where it was cooler and dark for about 3 weeks. Brought him up, and now he's pounding minnows again. Point is...If he's otherwise healthy, a month or even two or more without food won't kill him.
Mine was just a yearling, and I could hardly believe that he could go that long and be OK. But he's fine. Good luck with yours. Duffy