Please note that ResearchGate is neither a journal nor a publisher, i.e. the upload of articles to RG is not regarded as publication. You should cite the original publication. However, if you want to cite unpublished research made available through RG, I recommend to indicate the author(s), the title of the text, and its URL (and pershaps the date when you accessed it). E.g., the URL of Data Hermann Carl Vogel's report on a tour to England, Scotland a...

(an unpublished translation of a German report) is https://www.researchgate.net/publication/380323383. The title added to the URL can be omitted to keep the URL short.