You are going to need a little more than fiberglass to fix a kit. This is a breakdown of some of what could go into a repair. It involves several step most of which I have left out, but can give you a good ex. if you want it.
Fiberglass can be used to build up material that is to be remove gradually through sanding etc. to get the original or preferred shape/fitment. Most of the time both the front, back, or sides of the area are worked to get contours back to spec. i.e. a 1 in. tear may require 4 in. of work area to repair & conceal damage.
An epoxy repair kit can be used on abs plastics to repair tears & small holes. Basically it is used the same except there are no mats like fiber glass.
Bodyfiller is used over either of the 2 above or alone & is sanded down before paint can be (re)applied.
Fiberglass is a "mat" coated with resin & hardener & is usually cut to shape before application.
Epoxy & fillers are pastes/liquids that are mixed with hardeners then applied.
If you do not have body experience do not try to fix a kit in the backyard. The resins & hardeners used on fiberglass, epoxy, & filler can make an even bigger mess than you've started. There really isn't a cheap way to get around bodywork. Make a friend in the bizz if you have time & try to pick up a few tips from them. Every shop does it a little different so you will get 50 ways to do it right each time.