This is a frequent problem in ResearchGate, most probably caused by an automatic algorithm. See "Another researcher has claimed my publications" in https://explore.researchgate.net/display/support/Authorship for instructions.
H have not clearly mentioned your problem of correct authorships if it is missing in article u have to contact with journal editor this is easy to correct if u have problem in RG it does not matter.
It is due to fact that your co-author neglected to include your name as a co-author when uploading the paper in research gate. As a result, it will not be seen.
Another possibility could be a mistake made by journal officials during the uploading process; please contact the editor.