The reason to knit a swatch in the round is because some people purl looser or tighter than they knit, so a flat swatch may not have the same gauge. You can fake an in the round swatch knitting flat though.
CO about 10 more sts than needed for the gauge, knit the first row, then slide the sts back to the beginning of the needle. Keeping the yarn loose across the back, knit another round. Repeat. That way you mimic knitting in the round, but can lay it flat to measure easier. The edge stitches will be very loose, so don't include them, just the stitches in the middle.