Because the front or right side of the work is always showing, you have to knit the opposite of what you would normally knit on the wrong side or back when knitting flat. In the case of Garter Stitch knit flat, you would work knit stitches on both the right side and the wrong side.
The opposite of knit is purl, so to knit Garter Stitch in the round you have to knit one round and purl the next round. That gets you the nesting ridges of purl bumps that make Garter Stitch look the way it does.
As with other textured stitches worked in circular knitting, you will get a bit of a phoney seam where you switch between knitting and purling at the beginning of the round. It's not a major thing, but you can see it if you're looking.