ResearchGate is a business that hosts open access research. It is neither a publisher nor a journal. It is a popular hub on the web for sharing academic publications. There is no editorial review board, nor does ResearchGate require that articles be peer reviewed, although they may be. Since it is an academic social network and there is no process for vetting the articles, evaluate each source carefully. If you choose to use an article found on ResearchGate, cite it using the citation information provided by the authors. No mention of ResearchGate is necessary.
Sources: Guide from the University of Michigan and the ResearchGate website.
