Glosses are diluted dyes. They produce less intense color than a full-strength paste. You may choose to make a gloss to stretch your paste and save money, or (most often) because you want to add less color than you get from a full-strength dye treatment. The color is still as permanent as ever; you shouldn't count on glosses to fade more than the dyes do when used at full strength.
Glosses are typically applied for a shorter time than full-strength dyes, usually somewhere between 5-10 minutes (to mildly tone down your orange hair with faster-acting dyes like indigo and buxus) and 1 to 2 hours (for glosses containing henna, or to get browner color from an indigo or buxus gloss). The longer you leave plant dyes on the hair, the more time they have to deposit color, though for each dye there is a point beyond which it won't deposit (much) more dye.
You might use a henna or henna/buxus gloss, for example, on light hair for 1 to 2 hours to get a lighter shade of strawberry blonde or light auburn respectively. A henna/indigo gloss applied for just a short time (30 minutes) can give a light brown color to gray hair.
A henna gloss can also be used to refresh or deepen your color if you find it has faded, although pure henna properly applied tends to fade very slowly, if at all.
You have a lot of choices for the base for your gloss. Many people use a cheap, not very rich conditioner like Suave, VO5, White Rain, Trader Joe's, because it is convenient and easy to mix and apply. You can also make a flaxseed gel, xanthan gum gel, pectin gel, cornstarch gel, or use any kind of yogurt, or instant pudding mix combined with water.
For a very dilute gloss, you might use 1 tsp of dye powder per cup of conditioner (or gel, etc). For a stronger gloss, you can use 1 to 2 Tbsp of powder.
If you want to make sure your henna has good dye release, you can add prepared, dye-released paste instead of powder. Putting henna powder into room temperature conditioner, you don't know for sure how quickly dye release will occur. If you want an ashier shade of blonde or brown from a henna/buxus or henna/indigo gloss, it might be desirable to get a weaker effect from the henna, in which case using it as a powder instead of paste may be helpful.