Please note that ResearchGate is not a publisher and not a journal, you cannot "publish" an article here. It is a platform for books and articles published elsewhere, or to present unpublished texts, data etc. See also "Is ResearchGate a publisher?" in https://help.researchgate.net/hc/en-us/articles/14292596164753. And see this help page ("How to add research") for instructions how to add published or unpublished research to ResearchGate: https://help.researchgate.net/hc/en-us/articles/14293005132305
You should search for journals that fit with your domain, then read their scope, publishing guidelines, and terms.
For example, you can check the journals that your colleagues in the same program you enrolled in for your PHD are in, read their guidelines, check their research articles, and then develop your own.