I use this medium all the time and it does sound like you used too much of it, either in one area of the painting? Or in all areas? Is this the case? I mean, is it glossy in patchy areas? As you are painting with it, you will notice it thins the paint out quite a bit. It is a lot like Liquin in this way, in fact, I think it was meant to be a non-toxic replacement for Liquin. That is basically how I've been using it--almost as a kind of quick-dryer, a kind of glaze to layer (because it thins out the paint and makes it much smoother) and these kinds of uses.
This is how I use it. AS I paint, I use very little of it. I put out a little dab of it next to the colors, dip my paint in it just a touch and pick up some paint color and use it as a smooth layer, depending on how opaque I want to cover. If it's too thin, I'll pick up more paint without dabbing it in the gel and cover over it a little more.
AS a general media for the entire color, I will mix the gel with all of the color (again, you do not need more than a small dab of the gel to an almost generous amount of paint on your palette). I do this when the paint color is particularly dense, dry, or sticky just to smooth it out so it's more flowy.
Just use it sparingly. If you see it's too thin in more places than others and are afraid of it shining, take a small amount of paint that is not dipped in the gel and lightly brush over it, but the less gel you use, the more likely you really won't need to do this.
Keep experimenting with it too and look closely at the differences in it while it's still wet to see the thin-ness of paint. Too much gel will most likely make it slightly glossy.
