How to add ResearchGate preprints manually to google scholar?
Google Scholar may well index your preprint on ResearchGate automatically after a few days, although it's rarely possible to be sure what Google Scholar will do, since it appears to run largely unattended.
You can manually add articles to your Google Scholar profile by going to your profile page and clicking on the plus icon just above the list of your publications:

Other people can cite anything you post to ResearchGate, although the citation format will depend on the citation standard (Harvard, APA, MLA etc) that they are using. ResearchGate says it issues DOIs for preprints, which make it easier to cite them.