An even better question would be, why would you WANT to get rid of Garter Snakes?(There's no such thing as a "Gardner" snake) They are harmless and generally docile, and they don't cause any damage to lawns, flowers, gardens or people or pets. There are no snake repellents that actually work, that won't kill or hurt children or pets. Natural snake predators, like King Snakes, can be a deterrent, but again, that's a snake, also. If snakes are hanging around a property, it's because something on the property is attracting them and keeping them there. Getting rid of whatever is causing them to hang around is the best option. Garter Snakes eat frogs, toads, and fish, primarily, so if you've got permanent water on the place that allows those animals to breed, then you will have things that show up to eat those animals. Draining wetlands or permanent water sources can get you in MAJOR legal problems with the Federal government, so you really do not want to go there. Snakes are also attracted to things that provide shelter and hiding places, like woodpiles, trash piles, stacks of old lumber or sheets of metal on the ground, dense bushes and shrubbery, tall grass...so keeping those things under control will minimize their attractiveness to wildlife-MINIMIZE, not eliminate.
What I do is throw moth balls under the house and a few in the closets, one under the bed, and under the sinks. This helped me. The house smells like moth balls, But better the having snakes in the house.
Pick them up then put them in the woods