Gesso (which is Italian for "gypsum" or chalk) comes in two kinds - acrylic and traditional - depending on the binder used. In Europe, the word gesso is reserved for animal-glue mixtures, while the modern product is called "acrylic polymer primer." This acrylic "gesso" doesn't create a chemical bond with oil paint, but a mechanical bond is formed if the proportions of acrylic and chalk are correct. It's helpful to wipe acrylic-primed canvas with a damp cloth to remove any residual surfactant from the surface (and let it dry) before applying paint. Acrylic gesso creates a flexible surface suitable for paintings on stretched canvas.
Traditional gesso offers a unique surface which is both smooth and absorbent. It is inflexible when dry, and this makes it an appropriate foundation for oil paint, which also becomes hard and brittle when dry. It's possible to use it on stretched canvas, but only if the canvas is subsequently mounted on a rigid panel. When painting outdoors, it's usually easier to paint directly on panels, which are sturdier and more compact than stretched canvas.
There are recipes for homemade gesso, but to make it easier and to avoid problems caused by inexact proportions, Gamblin's Traditional Gesso is what we recommend.
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