Gesso is a white paint with a powdery consistency. (It reminds me of tile grout.) Its principal use is as a primer on canvases, mountboard, or hardboard - where it acts as an undercoat creating a great surface to use acrylic paints on.
However, in mixed media paintings, it is interesting to apply it with a palette knife. You can achieve a variety of effects that suggest a range of different textures.
On this small canvas, I applied it in smooth strokes to suggest the window frame; working more roughly it is used to suggest foliage. The edge of the palette knife was used with it to indicate grasses and stems.
This vibrant depiction of a 'South Coast' has added interest and relief. The paste is used to show the rocks and craggy cliffs. It is also used in the foreground to suggest vegetation. Once dry, acrylic inks and pastels were added. See how the 'valleys', grooves and peaks react with the inks creating fascinating patterns and tonal effects.