You could make hundreds of amigurumi and never once measure the gauge.
When do you need to worry about it?
One – if you want your finished project to turn out the size the pattern says it will be.
Two – if you have exactly the amount of yarn specified in the instructions and you don’t want to risk running out.
So how do you measure it?
Well, because you’re stitching in the round it’s a little different than other gauge instructions.
Crochet up a swatch of five rounds. Make sure you’re stitching through the back loop if that’s what the pattern says to do. (All FreshStitches and Shiny Happy World patterns are stitched through the back loop.)
Ch 2 (I like to start with a sloppy slip knot.)
Round 1 sc 6 in 2nd ch from hook (6)
Round 2 sc twice in next st (12)
Round 3 [sc twice in next st, sc in next st.] 6 times (18)
Round 4 [sc twice in next st, sc in next 2 sts.] 6 times (24)
Round 5 [sc twice in next st, sc in next 3 sts.] 6 times (30)
Now – measure across the diameter of your swatch.
Mine measures 2 1/2 inches.
If yours is 2 1/2 inches – good! Unravel your swatch (so you can reuse the yarn) and start stitching.
If yours is a little small, try using a larger hook size. If yours is a little big, try going down a hook size. You can also try switching between different hook materials – a bamboo hook “grabs” the yarn a little more than a metal hook, for example, which can change your swatch size.
Don’t try stitching just a little tighter or a little looser. As you work you’ll tend to revert back to your natural tension, so that’s not a good way to get gauge.
Keep experimenting until your swatch matches the one in the pattern – then stitch away!