I write this from memory.
In the early days of environmental management system, we used to refer to BS 7750 and EMAS. Both required a review of the environmental situation of the organization, including past accidents, incidents, licences obtained, disputes with stakeholders etc., before starting to work on the EMS. Essentially the environmental review is to know (and capture all relevant information available not only in documents but also anecdotal) the condition, and potential liabilities and issues, before embarking on setting the EMS.
A gap analysis is done against a reference standard (e.g. ISO-14001) and gaps between what has been achieved and what is required under each operating section is captured in a gap analysis.