You could probably just start your project and measure as you go. But if you want to do a gauge swatch, there is a way to do a flat one that will give you the same tension as what you’d knit in the round. (You will actually get a different gauge than knitting flat because your stockinette is all knit stitches in the round, but alternating knit and purl rows when knitting flat).
The advance techniques video page actually tells how to do the gauge swatch, but I will quote it here:
“Thankfully, there is a trick to working a gauge swatch for knitting in the round. You knit a flat stockinette sample using all the knit stitch by doing the following: *knit a row; slide the work to the other end of the circular needle or DPN; leaving a long strand of yarn in back. Repeat from *. If you leave the strand in back long enough, you can use it up by knitting with it for a row. This removes the strands from dangling behind the swatch, but is entirely optional. In any case, the end of the row will be loose and messy, so add about four stitches to your swatch to accomodate this.”