Many a time it has been observed that RG contain data, unpublished Articles/preprints which are not published in any other journals/online site. If these article contain only DOI of Research Gate can be be cited ?
Yes. RG has over the years grown to be one of sources of full text articles that appear among the first results in Google search. Because such documents have persistent DOIs, they can be cited.
Yes, you can use information from a preprint. Because there will likely be only formatting or small editorial changes, you can cite it as the official published version.
The ethical key is to not misrepresent the publication-status of a particular work. In an age when one's employment, scholarly reputation, or financial prospects hinge on publication, there is a temptation to cross scholarly ethical boundaries with premature citation or claiming faux publication status. So I would answer the question with a qualified "yes, but . . . ."