Can I still publish a paper in a journal after it has already been assigned a DOI through Zenodo or ResearchGate? Zenodo and ResearchGate can assign a DOI to an article. After publishing an article on Zenodo or ResearchGate with a DOI, if the article is good, will the article still be accepted for publication by peer-reviewed journals?  Assume that the journal in question allows green open access (i.e., posting of a preprint on a public site).
To "publish" something really just means "make it available to others". So, there are many ways to publish something: You can strive to get it accepted in the most prestigious journals of your field, or you can just put it on your website -- strictly speaking, both qualify as "publishing". Putting an article on Zenodo, ResearchGate, or arXiv is closer to the latter than the former; the fact that some of them can give you a DOI really just means that you make it easier for others to find and reference what you uploaded.

From a practical perspective, the difference between all of your options is how much vetting and endorsement the locations where you publish provide for your work. Academic journals review your article, and this provides a level of endorsement that lets others trust that what you published is useful and correct. On the other hand, putting an article on a web site or ResearchGate involves no vetting and, consequently, does not bestow on you the prestige that comes with the vetting. If you wanted to use your "publications" as part of a job application, it is this prestige you want, and consequently, putting your work on a website of your own will not be enough.