See this help page ("How to add research") for instructions how to add published or unpublished research to ResearchGate: https://help.researchgate.net/hc/en-us/articles/14293005132305-How-to-add-research. You may select "Preprint" as type. But note that this will not be a publication. ResearchGate is not a publisher, you cannot "publish" an article here. It is a platform for articles published in journals, or to present unpublished texts, data etc. See also "Is ResearchGate a publisher?" in https://help.researchgate.net/hc/en-us/articles/14292596164753.