The owners and administrators of researchgate, insist on the appropriate blocking of indepedent scientists researchers and other scholars with published scientific articles and simultaneously have none official collaboration with some universitiy or institution. Although the website of researchgate offers the selection to join on the community as an indepedent researcher without institutional e-mail adress, if you try to register with non-institutional e-mail by submitting your CV and reporting links of your articles and papers, you will soon find that the activation mail from researchgate will never come in your inbox.
Practically, my conclusion is that researchgate does not accept independent researchers and the selection of registering as an independent researcher is only available for fictitious reasons.
Essentially, a "private club / private community" of scientists and researchers has been created due to the particular practice. Researchgate prohibits other independent researchers to join in the community. This is unacceptable from me.
I would suggest in all independent resarchers scientists and scholars to condemn this questionable tactic of researchgate.
Related dialogs and thread.
https://www.quora.com/ResearchGate-Im-a-freelancer-who-wishes-join-to-ResarchGate-but-signing-up-it-requires-a-institution-email-What-can-I-do-without-an-institution-email-address