Green open access refers to articles made available through open access institutional repositories or personal archives. These articles, often pre-print or post-print but usually not published versions (unless the author's publisher allows it), are submitted by the authors to the repository or archive, and can be subject to publisher embargoes. If an institution or organization mandates open access availability of its research, that OA is usually achieved through repositories.
Gold open access is on the journal level, referring to journals published open access online and freely available. These journals, such as PLOS One, are peer-reviewed like traditional journals, and publish articles immediately. A list of OA journals can be found at the Directory of Open Access Journals (DOAJ).
Hybrid open access refers to the way traditional publishers have integrated Gold OA into their publishing models, in which authors, for an additional fee, can have their articles made open access, usually after an embargo period.
A list of publishers that allow authors to deposit their articles in repositories and how long, if any, their embargo periods are can be found at SHERPA/RoMEO: Publishers Allowing use of their PDFs in Repositories.
"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
Libre open access: no cost to read (gratis) but includes more reuse rights, usually by way of a Creative Commons license, and thus fully in line with the Berlin Declaration on Open Access to Knowledge in the Sciences and Humanities (2003):
"The author(s) and right holder(s) of such contributions grant(s) to all users a free, irrevocable, worldwide, right of access to, and a license to copy, use, distribute, transmit and display the work publicly and to make and distribute derivative works, in any digital medium for any responsible purpose, subject to proper attribution of authorship (community standards, will continue to provide the mechanism for enforcement of proper attribution and responsible use of the published work, as they do now), as well as the right to make small numbers of printed copies for their personal use."
The distinction between "free to read" and "free to use" was developed by Peter Suber, one of the leaders of the OA movement, in a 2008 newsletter you can read here.
Based at the University of Nottingham and in association with JISC, SHERPA's databases support authors and researchers in need of information about publishers' and funders' open access policies.