If you mean how to add a journal's title to ResearchGate's database, this is not possible. See my reply to this question: https://www.researchgate.net/post/How_can_I_add_the_name_of_the_journal_where_my_article_is_published_when_the_name_of_the_journal_is_automatically_cancelled
If you mean how a journal can be indexed by ResearchGate, see my reply to this question: https://www.researchgate.net/post/Aoa_Sir_I_want_to_our_research_journal_indexing_with_ResearchGate_kindly_give_me_detail_about_that