I was confused for a long time about how to sew up Garter Stitch. I knew how to do mattress stitch on Stockinette and other similar stitch patterns, but I wasn't sure how to translate that to fabric that was all bumps.
The truth is, though, once you see how to do it, seaming Garter Stitch is just as quick and easy, if not more so, than seaming Stockinette. Just as with Stockinette, though, you need to be sewing pieces together that have the same number of rows, and you have to be careful to keep an even tension in your sewing yarn or thread to ensure that the seam doesn't pucker. But other than that it's really quick and easy to seam up knitting worked in Garter Stitch just as it is with other basic stitch patterns.