Gloss mediums make paint thinner and more transparent, which is useful if you want to paint thin but brilliant glazes. Use them to enhance the natural sheen and luminosity of acrylic paint.
Matte medium gives a more subtle, non-reflective finish. You can also mix gloss and matte mediums together in equal parts to achieve a kind of semi-gloss.
Gel medium thicken your paint so that it retains brushstrokes, which is great for impasto techniques. It also enhances the adhesive properties of the paint, which makes it suitable for collage work.