Often you can tell what a word means simply by seeing it in context; other times you'll need to check the glossary. If you were reading about house painting, for example, you might find this definition for gloss listed in the back of the book: "A surface shininess or luster; polish or sheen." Paints are usually designated according to such a scale (gloss, semi-gloss, or flat). But some manufacturers take it even further in terms of nuance, bandying about such sensual-sounding but mystifying descriptors as silk, suede, eggshell, platinum, pearl, melamine, velvet, and satin. You may not find good definitions for these words on the back of a paint can, but when it isn't enough to just say "gloss," you can always consult the MPI standards. Glossay appears five times in OhioLINK, Glossry another five times, and Glosary ten times, making them all semi-glossy typos of low-to-moderate probability.
(Peeling paint at a New York City subway station, from Wikimedia Commons.)