Garter Stitch is all knit stitches when you work it, but the effect in the knit fabric is that you have knit rows followed by purl rows, since the opposite of a knit stitch (and therefore the back side) is a purl.
This makes picking up dropped stitches a little more involved than it is when you're picking up a stitch on Stockinette fabric, where you have all knit stitches facing you.
So in order to fix a dropped stitch in Garter Stitch, you first need to be able to read your knitting and determine if you're on a knit row or a purl row where you need to start picking up. If you're on a knit row you'll see the Vs of knit stitches; on a purl row you'll have bumps.
In the example shown a knit row is the first row where the stitch needs to be picked up. To do it, first work to the stitch that needs to be picked up. Then make sure the strand of the ladder that's going to be picked up is behind the loop of the stitch.
Take a crochet hook through the loop from front to back. Grab that lowest strand and pull through.