It should have other ones that cover petrification by breath attack, so it does protect against gorgons, which is why its called Medusa's Bane, its a bane for Medusa who is a gorgon.
There isn't an immunity specifically against Petrification in its entirety but the tag of the item could be added as a check for petrification spells to not activate.
This would probably be the better option, as changing the item to be immune to an array of specific spells would vastly increase its value, and there are spells like Prismatic Spray which only petrify at random alongside beholder eye rays.