The phrase generally points to an unrefined-looking grey sea salt made through shallow salt-marsh evaporation. The crystals pick up their grey tone through contact with clay-lined beds and retain natural moisture after harvest.
Shopper terminology varies. Celtic Sea Salt can also be associated with a specific branded product, while French Grey Salt is a descriptive category tied to moist grey salt from French salt-marsh traditions. They are close enough in everyday search language that shoppers often compare or substitute them by texture and use.
On this site, French Grey Salt is the relevant product. This page explains the category and sends every shopping action to that existing product rather than creating a second name for the same purchase intent.