Does an unpublished document you upload to ResearchGate count as prior publication when submitting to a journal? On ResearchGate, if you upload something like a thesis, and then it says publish resources, does this mean the thesis becomes properly published in the sense of the usual requirements of journals that there be no prior publication of the submitted work? 

I ask because I’m drafting thesis chapters into journal articles and did not choose to publish the thesis as a whole work in order to do so, so I want to make sure that by clicking publish resources I am not actually publishing the copy I’ve uploaded, I’m just making it publicly available? Does this mean it becomes published despite it not technically being a published work? (If so, I’ll just remove the copy I uploaded).
The problem is less ResearchGate and more your field. As far as I know, ResearchGate, unlike Academia.edu, does not claim copyright or commercial license for your uploads; and they refer to their upload service as more self-archiving than anything else. That said, different fields and different publishers have different attitudes toward what it means to be prior publications. 

In Mathematics (my field), for example, almost certainly no journal will count uploads to pre-print repositories or online social media or your own webpage as prior publication. 

In fields like Medicine and some subfield of Biology, the rules are much stricter, where sometimes too-detailed conference posters can count as prior publication when it comes to journal submissions. Wikipedia has a listing which shows, e.g., journals of the American Society for Microbiology having this type of strict pre-publication rule. 

So the only general advice for your question is: consult the website of the journal to which you intend to submit your articles.