Gold open access (full OA) is a journal where all articles is openly available. The journal can bre read free of charge by everyone, but some journals charge an article processing charge (APC) of the researcher for publishing OA. Diamond OA is a full OA journal free of charge, no APC:s and no subscription fees are charged.
Check the quality of the journal when selecting publishing channel for an article, if the publishing channel is previously unknown. The reliability of open access journals can be assessed in the same way as for traditional scientific publications. By doing this you can avoid so-called predatory publishers (vanity publishers). At least these should be checked:
More about evaluation of open access publishers:
Open access publishing of monographs may be possible, but it is still rare compared to open access journal articles. Some publishers have an option for gold open access, but the book processing charge (BPC) is considerably higher than the APC for journal articles.