Green open access (also called self-archiving) refers to making a version of work (usually the author's accepted manuscript) available in an open access repository.
Depending on the journal or publisher, there might be different conditions attached to the green route, such as which version can be used (often the accepted rather than the published version) and an embargo (a time during which the manuscript cannot be made publicly available after publication).
Many funders recognise green open access as a route to complying with their open access requirements, although they are likely to have expectations around embargo periods and possibly also about licence types.
Placing work in ResearchGate or Academia.edu is not considered to be green open access and is not usually allowed by publishers. You should always check your publishers policy and your route to funder compliance. You can check publisher policies by using Sherpa/Romeo, and you can also contact the Open Access Team.