See these discussions with several recommendations:

https://www.researchgate.net/post/What_are_the_procedures_for_publishing_a_research_paper/1

https://www.researchgate.net/post/For_publishing_article_in_any_journal_which_necessary_steps_should_be_taken_care_by_a_new_researcher/1

If you mean to upload a paper to ResearchGate, see this help page ("How to add research") for instructions how to add published or unpublished research to RG: https://help.researchgate.net/hc/en-us/articles/14293005132305

But note that ResearchGate is not a publisher, you cannot publish a paper here. It is a platform for papers published in journals, or to present unpublished texts, data etc.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.