Gabiden Konisov re: Is it possible to cite publications in Researchgate if they are uploaded manually? Certainly publications on can be cited.
Any source of information or reference to a written or spoken work (or anything else for that matter!) can be cited or referenced, and must be if you are quoting or referring to it as an authority.
If external publication information is given then that is included in your citation - best practice is to go to the published site and verify, even if the item can be downloaded on ResearchGate.
If the item is only available on or from ResearchGate, then you can either consider it as 'published' on RG or cite it as 'work in progess' / 'unpubished work' with the year of production (i.e., when the paper was written or made available).
If you contact the author then you can cite 'personal communication.'
However: The academic authority and validity of material cited can [as it should be] questioned. It stands to reason that unpublished / in progress / self-published work is more likely to be questioned by your peers and professors.