100% No doubt female.

It's funny, when I first saw the title of this thread, I thought, you're in the wrong forum

You see what a difference it makes when we can see the last 1 or 2 thirds of the snake? Now there's no doubt. Believe me, when (if) you get a male and have them side-by-side, you'll see there's never any reason to probe a garter snake, and the difference between the sexes will be obvious.