You don't have to be a special effect makeup artist or have expensive materials to achieve a great looking gash.
If you want to make a large scar, or one that has yet to heal, you will need to do a bit more work, but the effect can be great. Let's say you want to make a gaping wound on your forehead. To do this, you will need:
1/4 cup of water, boiled
Skin astringent or rubbing alcohol
A cotton ball
some large, clean popsicle or craft sticks
cream or liquid base make-up in your skin color
Ordinary loose make up powder to match your skin tone (if you only can find pressed powder, simply break it up)
Some deep red blush (nothing sparkly or shimmery)
A large, clean, soft make up brush
A small, clean, soft eyeshadow applicator brush
A make-up applicator sponge
Pull all your hair back and make sure that it is out of the way. Clean the skin on the area you will be applying the scar and dry it well. Then, using the cotton ball, apply some skin astringent or rubbing alcohol to remove all traces of skin oils. Dry well again. It is important to prepare your skin first, because the next steps are time sensitive.
In a bowl or cup, mix the 1/4 cup of boiling water with the unflavored gelatin using a craft stick. It should get very thick. Let the mixture cool slightly, but just until it is warm enough to put on your skin. The cooler it gets, the more clumpy and difficult it will be to work with. If your skin became moist from any steam from the water, dry it again.
Go before a mirror and apply a thin layer of petroleum jelly to your eyebrows with cotton swabs. This is so that if any of the mixture gets on your eyebrows and dries, you won't pluck your hairs out with it when you pull it off.
Using a fresh craft stick, scoop out a teaspoon-sized amount of gelatin mix. You are going to apply it in two layers; first the lower, then the upper. You will want a slight gap between them to indicate a gash.
Tilt your head back and, halfway between your eyebrows and hairline, apply the lower layer by slowly drawing the stick across the area you wish to inflict. Don't smear it; just let it ooze off the stick. Keep your head tilted back a couple of minutes until it sets. Using another teaspoon-sized scoop, tilt your head back again and draw it across above the first layer in a very slight arch, so that the ends of the layers meet, but there is a slight separation in the middle. You can press the long edge of the craft stick gently into the gash and, with a delicate hand, rock it back and forth if you would like to make it more gaping. Allow it to set.
Give the mixture a couple of minutes to set better. Then, using an applicator sponge and the base make up, very gently pat some color on it and blend outward onto your skin. If there are any nooks and crannies that need filling, try using a cotton swab, but very gently.
Once the scar matches your skin tone, use the large blush brush and the loose face powder and apply powder to the area by tapping and dusting (did I mention you want to be gentle with it?). Use the deep, red blush and the small eyeshadow brush to redden the inside of the gash. If you like, use some brown eyeshadow deep in the crevice to give it more depth.
You should have a very nice, deep, gashed scar wound on your forehead now. The same technique can be applied to just about any other part of your body (arm, leg, neck). It's not a difficult technique, but if you are unsure of your make-up skills and have the time, you can always practice ahead of time to perfect it.
If you want to really gore it up, get yourself some thick, black thread and a fine needle. Thread the needle at knot the end. Then, very carefully, looking in the mirror, poke the needle up through the two layers and continue to stitch your "wound" together, gently running the thread through but not tugging it tightly. Just make sure not to poke yourself (work upward so you don't come too close to your eyes). Then, mix a tablespoon of corn syrup, with enough drops of red food coloring to make it a deep red, and a drop of liquid dish detergent. Using a craft stick, dip it into the fake blood and simply dab it here and there between stitches, allowing a tiny bit to drip down over the scar. Just a little goes a very long way; too much blood will simply cover over the neat effect you just created.