Gauge for circular knitting can be different because you only knit for stockinette--not knit one row and purl the other. You can do a gauge swatch without casting on enough for the round by using a dpn or circular needle, knitting across, and then sliding the stitches back to the other tip, leave a long loop of yarn across the back, and knit across again. keep doing this until you've got enough to measure gauge.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
When you knit regular stockinette (the sort of pattern you usually see, with little "v"s on one side of the fabric and bumps on the other) on straight needles, you have to knit one row, then purl the next. Most people have a slightly different gauge for their purl rows, but it all works out because of the back-and-forth going on.
When you knit in the round, you go round and round and round on one side of the fabric, just knitting, never purling (unless you have ribbing). Since there are no purls, your gauge can be different for knitting this way.
A way to knit a gauge swatch without doing enough to make a full round tube is this: cast on 20 or so stitches on a double-pointed or circular needle. Knit a row. Then, instead of turning around and knitting or purling back, slide the stitches to the other end of the needle. Your working yarn is now at the "wrong" end of your knitting. Pull it loosely across the back, and knit your stitches again. Keep doing this -- knit a row, slide stitches over, pull working yarn loosely across the back. The front of your fabric will look like normal stockinette stitch; the back will be a mess of long strands, but you won't care -- you're measuring for gauge on the front side.