Many journals are missing in RG's database. See https://help.researchgate.net/hc/en-us/articles/14293139566353: "Due to resourcing constraints, please understand that we cannot accept requests to manually add journals to our database. Currently, we only import journal data from CrossRef. ..." - When a journal is missing in RG's database, I add the bibliographic data to the abstract field (like in https://www.researchgate.net/publication/268925009 or https://www.researchgate.net/publication/344474227).
See also this help page ("How to add research") for general instructions how to add publications to ResearchGate: https://help.researchgate.net/hc/en-us/articles/14293005132305