Note from Les: In this case, the hinges were door ones.  A door hinge is ok for a regular door.  Sometimes you see doors with three or four hinges.  The reason there are more is that the door is very heavy for it's width and so 3 or four may be needed to distribute the weight more evenly across more of them.

When it comes to a gate, however you can use a beefier hinge and then only two will be required.  But you should not use a door hinge on a heavy gate because in most cases it will not be able to support the weight and the abuse a gate gets.