Henna gloss is something that you make yourself, not buy. Its basically BAQ (body art quality henna) mixed with water or tea to activate and conditioner added to it. There are so many threads here and online in general about how to make and apply henna glosses.
Henna gloss is usually a DIY process of blending henna with a moisturizing conditioner. The only vendor that I know of that sells henna gloss is Shi-Naturals. She uses BTMS rather than conditioner which is one of my favorite ingredient for slip and moisture. Here's the link to her gloss http://shinaturals.bigcartel.com/product/henna-gloss-thickening-strengthening-conditioner