Garter stitch or stockinette stitch is the result of working the sts a particular way, not how you do the stitch. Knitting in the round you're working on only one side, not turning front to back. So when you knit every stitch on all rows in the flat, you get garter stitch. When you knit every stitch on all rounds circularly, you produce garter stitch. For garter stitch in the round, you would knit one round, purl one round. When doing that flat, you produce stockinette.