It is hard to tell from the picture, but if you even think it might be a protected species, I would not even think twice. If it is listed as a threatened species, but not protected, I would still release it since I would rather leave the wild garter in the wild. There are more then enough captive breed garter now a days, and more are relatively inexpensive. If you have only had it a couple of days, it would be better to release it and purchase a captive breed garter, then risk keeping one that is threatened or protected. Just my two cents!
Jason