"Gratis" and "libre" both mean free (for more on their etymology, see Wikipedia), but within the OA world and the larger open culture movement, their meanings are a little more specific:
Gratis open access: no cost to read, but reuse is restricted
- Permission is required for uses other than reading and what's covered by fair use.
Libre open access: no cost to read but includes more reuse rights
- Removes "price barriers and at least some permission barriers" (Suber, 2012, p. 66). Usually by way of a Creative Commons license.