In gas flush, a machine pumps nitrogen into packaging. Nitrogen pushes oxygen out of foil pouches or some other flexible packaging. This technique is used in two cases - (a) the product is delicate, (b) the product is easily oxidized. Potato chip packaging are gas flushed.
If the barrier film of a pouch is sturdy enough, you can vacuum package your product. The catch is, the packaging does not look attractive. But this is our subjective opinion.
Oxygen absorbents are little porous bags filled with iron flakes or some other material. They are placed inside a packaging along with your product. Their job is to absorb the remnants of oxygen that have stayed inside the packaging. You can find them in a beef jerky bag, cookies bag, and in other food pouches packaging. Of all the four methods, this one is the most commonly used. If you are going to use oxygen absorbers, we recommend Multisorb.