Gauge is determined by the tightness or looseness of your stitches, and affects the finished size of your project. If you are concerned about the finished size of the project matching the size given, take time to crochet a small section of the pattern and then check your gauge. For example, if the gauge called for is 10 dc = 1 inch, and your gauge is 12 dc to the inch, you should switch to a larger hook. On the other hand, if your gauge is only 8 dc to the inch, you should switch to a smaller hook.
If the gauge given in the pattern is for an entire motif, work one motif and then check your gauge.