The label was right!
When I'm swatching with a nubly yarn like this Artful Yarns Legend, I don't measure a four-inch square for gauge. Instead, I remember how many stitches I cast on (20, in this case) and I keep track of my rows (40). Knit with US8s, this swatch measures six inches wide by eight inches long, in garter stitch. Therefore my gauge is 3.3 stitches per inch and 5.75 rows per inch in garter stitch. Very close to the label: 3-3.5 stiches and 5-5.75 rows per inch.
Time for a little math before I begin my hat.