If you're actually knitting in the round where you only knit on the outside... knit stitches should produce stockinette. You'd have to alternate knit rows and purl rows to get garter. UNLESS you are going from the inside of the loop to the outside and only knitting or you are actually knitting INSIDE the circle in which case you'd be getting purl stitches on the outside which would look like garter, too.
http://community.knitpicks.com/notes...g_in_the_Round