How much do we have to pay for Article Processing Charge in this journal?
If you mean ResearchGate: This is not a publisher and not a journal, you cannot publish a paper here. It is a platform for papers published in journals, or to present unpublished texts, data etc. See also "Is ResearchGate a publisher?" in https://explore.researchgate.net/display/support/Getting+started and https://explore.researchgate.net/display/support/How+to+add+research.
Nunuk Nugrohowati Thank you very much for raising this important point.
Wolfgang R. Dick has clearly explained what actually ResearchGate is.
Contrary to the general belief, this is a type of Social Networking platform where Scholars exchange information about
1. Their papers published in other Journals.
2. Exchange information about research they are involved in, by uploading preprints of their unpublished work/data.
The terminology used by ResearchGate is somewhat confusing when they show even your preprint as a 'publication' under your Profile.
Obviously, there is no APC for uploading your published or unpublished papers/data on ResearchGate.
ResearchGate is simply a repository of your research work for sharing with other scholars using this platform.
Consider use of this platform cautiously as some Publishers do not accept preprints those are already available on the internet.
Hope this helps.