In all of the journals, the unpaid articles are not kept under open access, so in order to get citation and to share his/her findings with the researchers the is it authentic/legal right if author himself upload his full paper at research gate?
It is actually even more complicated than that. Yes, you have to retain rights to be able to share openly on research gate. Whether the final authors version suffices to meet more restrictive permissions depends on the specifics of the journal. Some of them would not allow even that.
However, there is another means of dealing with this. Most journals indeed allow one to distribute copies of one’s work to colleagues upon request. Research gate allows for that request, in the form of the storing the paper online, but not opening it completely. Some have even argued that touching the download button could be construed as meeting that request requirement, even if we are not directly and individually in contact.
So, the long and short answer is that it is complicated. Many of us feel that we should air on the side of open this, and that it is easier to apologize then to ask permission. It is more important that the information be out there and shared openly.
There is no clear-cut yes/no answer to this question. The answer depends on the copyright transfer agreement (if any) the author signed. It may also depend on the version of article posted to RG (whether it is the final published version from a journal or, say, a preprint).
Also, there are diamond open access journals where the authors do not have to pay for publication but the articles are open access.
If copyright of the published paper is with the authors, then only they can upload the published paper in RG. Otherwise, they can upload the accepted version (author version).
It is actually even more complicated than that. Yes, you have to retain rights to be able to share openly on research gate. Whether the final authors version suffices to meet more restrictive permissions depends on the specifics of the journal. Some of them would not allow even that.
However, there is another means of dealing with this. Most journals indeed allow one to distribute copies of one’s work to colleagues upon request. Research gate allows for that request, in the form of the storing the paper online, but not opening it completely. Some have even argued that touching the download button could be construed as meeting that request requirement, even if we are not directly and individually in contact.
So, the long and short answer is that it is complicated. Many of us feel that we should air on the side of open this, and that it is easier to apologize then to ask permission. It is more important that the information be out there and shared openly.
In some journal publishers there's this new feature that allows the authors to spread their work(via internet link) to their network and colleagues for few months, free and open access i.e. Elsevier has "Sharelink" where everyone will have access on a specific paper for 50-days https://www.elsevier.com/authors/journal-authors/submit-your-paper/sharing-and-promoting-your-article/share-link. But, I agree on the above comment, archive your work in RG and only provide a copy upon request (it is also a good way to personally communicate to the authors and readers).
Also, many Springer journals let you share your article via ReadCube so that it can be read by anyone (but cannot be downloaded if one has no valid subscription) who has a sufficiently modern browser to handle enhanced PDF; for example the published version of my recent article Integrable (3+1)-dimensional systems with rational Lax pairs can be read in this way here: http://rdcu.be/Dkwh .
This can be a reasonable substitute for open access, but again you should check this for each journal separately whether this feature is available for it; see also https://www.springernature.com/gp/researchers/sharedit for details.