Gauge is not that important with amigurumi. Big or small they will be cute no matter what! If your ami comes out larger or smaller, simply adjust the size
One more little suggestion I have for you before you begin crocheting:
For most of the yarn, I prefer to use Elle cotton yarn. It is an acrylic based yarn which is thicker and sturdier than many of the other yarn brands out there. I would highly recommend not mixing brands of yarn for the various colors needed in this pattern.

Rule of Thumb: softer, thinner yarns will make your ami smaller in size; rougher, thicker yarn will make your ami larger. Just keep this in mind and be consistent.
The toy must be crocheted with tight stitches, to be sure that there won’t be any holes through which stuffing material can be seen.