How to set up a gear mate ?

Go into mates. By default, on the left, you'll see the standard mates. Go into the mechanical mates pulldown. Choose the gear mate option. You'll have to select two circular references to create the mate between, for example you can choose the pitch circle on each gear. By default the ratio will be set based on the diameter of the circles selected but you can manually change this ratio in the provided boxes.

First decide what your gear ratios are. If one is half the size of the other, it will have half the teeth, and the gear ratio will be 1:2. Go to your assembly>mates>mechanical mates drop-down. Now for the mates that go in the top box of the mates menu you want to select the temporary axis in the middle of each your two gears. These can be seen under View>temporary axis. Now you can select the gears mate. Plug in the gear ratios in the correct places and hit the check mark. (Note: you make have to manually align your gears so the teeth are exactly between each other before you apply the mate so that there are no collisions.) I have attached a simple example. All you have to do is turn one gear to turn the other.