Determining the best gate location(s) for molds is usually a matter of expertise and experience. SolidWorks plastics can assist you with automatic placement of the gate location(s) while in the “Gate Selection” PropertyManager.
When applying gates, click on the “Add Gates Automatically” icon 
and define the number of gates to be added. The utility will then populate the gate location(s) on the part and in the selection box.
This utility will not take the place of an experienced mold maker but will give you a best guess based on the geometry of the model keeping the “Melt Front” as balanced as possible.
