Breathing Color Timeless Gloss, diluted 4 parts Timeless to 1 part de-ionised water. You can add a drop of Photo-Flo if you wish, for improved wetting.
Spray it onto printed matte paper with a HVLP sprayer. 5-6 coatings for high gloss, allowing each one to sink in, but not dry, before applying the next.
For extra-smooth results, after applying the final coating (which you may want to make slightly thicker than the others), sandwich it face-down against a sheet of teflon-coated glass. It will take on the ultra-smooth surface of the glass, and, due to the teflon coating, won't stick to it and will peel right off.
Works better for some papers than for others.