It depends on copyrights. If the journal is open accessed, it is legal to float a copy on researchGate. If someone has given a private copy for his/personal use only, he/she can't add that on researchGate. If he/she do so, will violation if copyright agreement and is illegal.Is ResearchGate a journal?
ResearchGate offers the ability to search and filter on a variety of topics: author, institution, journal, publication, and so on. Members can request a copy of a paper from the author if it is not freely available. Full text publications uploaded to ResearchGate are indexed by Google.What is a research publication?
A publication is defined as anything that has been published: a book. a research paper, a news article. A research paper describes the output of research. If it is published in a Journal or conference, then it is a published research paper, or a publication.