Do users need an account (on Research Gate) to view papers on the site? Or, are these papers available to anyone who finds the page for a paper (i.e., through Google Scholar)? Doing some investigating with different browsers and computers is a bit inconclusive to me.
In my experience, the papers are often not online, but RG let's you "request" a paper, meaning the website sends a canned email to the author, which they may or may not respond to. This is regardless of whether or not the user has an account. RG infamously listed authors with their publications who never registered, and these would thus receive all sorts of spam. I'm not sure if that is still the case.
It seems to depend on which version of the paper is uploaded. Google Scholar appears to no longer index pre-prints on Google Scholar (h/t user2768) but it does index papers that are the final, published versions. These final published versions are available both via direct links and via Google Scholar search results without the need for a ResearchGate account.