I'd like to post a manuscript on ResearchGate for some feedback from peers, but wonder if journals will then consider to have already been published and refuse it. Does anyone have clarity on this, please?
I am not sure about the life sciences but in mathematics refusing publication because the preprint is online is very hard to imagine, and if a journal behaves in such a fashion it is probably better avoided anyway.
The version you submit to a journal should not be the same as what you post pre-print for feedback. Be able to show, if asked, that the preprint version was a first draft or working copy, and that changes were made before submission for peer review.
However, to be honest, I am not sure that I would share my drafts this way. There is too much chance that someone else might appropriate the work, or at least the ideas.
To be sure, read the guidelines for authors of the journal to which yyou plan to submit and see if they say anything about preprint drafts.