In the late seventies I kept about a dozen Gartersnakes (T. s. sirtalis, T. s. parietalis, T. radix, T. sauritus nitae) in a very large terrarium.
In this also lived a few lizards (Podarcis sicula) that were dumped at my place.
This all went well. The lizards were always basking on top of the snakes
When I gave the lizards grasshoppers or crickets or mealworms, there were often one or two garters that caught one of the insects and actually ate it.
But I do not consider it as "normal" food for Gartersnakes.
But I can imagine that they eat them in the wild so every now and then when there is nothing else to find.