Gravel is probably one of the worst substrates because it is non porous... So when a snake "goes" the liquid and feces runs between the gravel but ends up not being absorbed or dried out. This creates a breeding ground of bacteria that will fester and continue to flourish. Gravel will retain all the moisture that goes down in it from misting as well and once again, it becomes a breeding ground for molds, fungi and bacteria that can potentially be infectious to you and your snake. Unless you are scouring that gravel twice a week, it is simply highly unsanitary. The moisture that it collects also means that your snake will be very prone to scale rot, and/or fungal infections of the skin/cloaca/eyes/mouth. Gravel doesn't really dry out, definitely not fast enough to be a good substrate choice for any snake. Garter snakes need a very clean, dry habitat to thrive, misting is ok every now and then the but moisture must dry up in an hour or two and can't be lingering. With gravel, that is not possible. Hope this helps! I wOuld recommend switching to a substrate that is absorbent, such as coconut fiber, Aspen shavings, newspaper pellet, repti bark... Etc. That way when the snake goes (And garters go a LOT) it is absorbed and dried out. Gravel and sand are big no-no's when if comes to keeping any snakes, especially garters with their semi aquatic habits and sensitive skin.