The answer can be found here: https://explore.researchgate.net/display/support/Journals it says things like “If your journal isn't visible in the drop-down menu, that means it's currently not in our database.” and “At the same time, adding journals to our database is not something we can do on a case-by-case basis. That’s why we currently only import journal information from CrossRef, via its Journal API.” and “Currently, we only import journal data from CrossRef. If your journal is added to CrossRef’s journal database and has a valid ISSN, it should eventually become part of our database. We appreciate your patience in the meantime.”
I agree with Eman, we have received many emails from various journals claiming to be indexed in Research Gate. So I think RG management needs to answer that question.