Doubt it will work since you are adding nothing to take away the gloss. It is my limited understanding that they make matte medium by mixing matting agents with gloss medium. So, to my thinking it will be glossy if you add water only, but, what the heck, give it a try.

NONE- it doesnt work that way….

As others have said, you’ll need to add matte medium to your gloss medium in order to come up with a satin or semi-gloss finish. Here’s a link to Golden’s technical data sheet on their matte medium products that may be of help:

http://www.goldenpaints.com/technicaldata/mattemed.php

Over thinning the medium beyond the recommended amount by the manufacturer may cause imperfections in the curing of the film, but it won’t change the resulting surface of the coat (gloss or matte).

For best results, apply uniform layers of whatever mixture of Gloss and Matte mediums or varnish you wish over the entire painting. This will ensure that all paint films reflect to exactly the same degree regardless of how they looked (matte or gloss) when first applied.

Also as a top coat, a varnish will add UV protection to the painting.

So if I had two medium mixtures, one 100% gloss medium, and the other 25% gloss diluted with 75% water, and I used them seperatly on two seperate paintings, the one with the diluted medium will look as glossy as the painting with 100 percent gloss medium added?

You could take it even further and say the diluted mixture was 1% gloss medium and 99% water, how could that be as glossy as 100% gloss medium. I would think there would be a point where the end result would look like a semi gloss or satin medium was used.
After the water evaporated, you would have a very thin gloss medium layer remaining, which I would think would be less reflective than a thick layer.

YES…………..doesnt matter how thin you cut it youll end up with a GLOSS finish. Further more if you cut it too much youll end up with a very runny mess, I would not recommend you cut you gloss with 75% water, I would definately not go further than 50-60% tops if that. I mix my own and I get to the consistancy of “pourable” and stop.