ResearchGate does allow .zip archives for supplementary material. There is a checkbox you need to click before you can upload them. Perhaps that was the issue? If you want the JSON text to be displayed on ResearchGate you will need to upload the individual JSON files. Another option would be to put the .json files in a GitHub.com repo and provide a link. GitHub gives you syntax highlighting for code, including JSON, which makes it easier to read.
Daniel Leuck : Thx for the answer. However, i do not find a checkbox. I go to 'Research' and to the publication/preprint for which i want to add the dataset. Then there is the link 'Add supplementary material' which then brings me to a page for selecting files for upload. Then it tells me 'Error: wrong file format' (for zip, tar, txt, json, etc.).
Erich Teppan Ah. I was going to "Add supplementary resources" in the more (...) menu. I'm currently traveling. I'll take a look at publication/preprint when I return.