Gauge is mentioned in the pattern. 4 sts per inch in st st, and 6 sts per inch for ribbing.

This pattern has no gauge. The yarn “suggested” (meaning: the yarn I used) is crochet thread. The hook I used is US size 00 (1.75 mm). The pattern is worked like half a granny square, and so gauge is really up to you. Just remember that if you decide to use a thicker yarn then I did, you will need a bigger hook.