Matte and gloss medium are similar in the following ways…

Both add a protective coat to your art. Both generally increase richness, and both change the degree of shine or gloss of your art. You can paint on top of both of these mediums once dry, and you can continue to apply additional coats of either gloss or matte medium in your painting process.

Matte and gloss medium are different in the following ways…

Adding gloss medium over your piece will enhance the colors, making them look richer. Gloss does also add sheen. It makes your art look shiny. It tends to make the darks a little darker and the lights a little lighter. For this reason, gloss medium changes the look, often for the better, more so than matte medium.

Adding matte medium creates a matte sheen. In other words, reduces shininess. The work does become a small degree richer, but for the most part the values remain the same.