Plastic tanks are used for everything, including carrying gas. The main problem with using a plastic gas tank is that the plastic is prone to cracking and splitting. Fortunately, cracks in a plastic gas tank are relatively easy to repair so you won't have to purchase a new one at the sign of every crack.
Use the sandpaper to sand the area around the crack. This helps to make the area smooth by removing jagged edges. Even if it looks smooth to start with, you will still need to sand as there could be edges that are not visible to the naked eye.
Cut your fiberglass cloth into a swatch large enough to cover the hole or crack in your tank. Place the cloth over the crack or hole.
Use an epoxy solution. Pre-made epoxy can be purchased at your local hardware store. Brush the solution over the cloth, making sure to get the edges as well.
Leave the tank with the epoxy alone for 20 to 30 minutes. The epoxy will make the fiberglass cloth harden and seal to the gas tank. This will make the tank liquid tight again.