I guess then... specify the Discontinued Status: Clearly mention that the journal is no longer in publication or has been discontinued in the relevant sections of your research publications or profiles. This will provide context to readers and researchers who come across your work.
Many journals are missing in RG's database, not only discontinued ones. See https://explore.researchgate.net/display/support/Journals for explanations. In case of a missing journal in RG's database, I add the bibliographic data to the abstract field (like, e.g., in https://www.researchgate.net/publication/344474227 and https://www.researchgate.net/publication/268925009).